Cleanup: Replace oldstyle struct hack with C99 flexible array members

The Linux 5.16.14 kernel's coccicheck caught this. The semantic
patch that caught it was:

./scripts/coccinelle/misc/flexible_array.cocci

However, unlike the cases where the GNU zero length array extension had
been used, coccicheck would not suggest patches for the older style
single member arrays. That was good because blindly changing them would
break size calculations in most cases.

Therefore, this required care to make sure that we did not break size
calculations. In the case of `indirect_split_t`, we use
`offsetof(indirect_split_t, is_child[is->is_children])` to calculate
size. This might be subtly wrong according to an old mailing list
thread:

https://inbox.sourceware.org/gcc-prs/20021226123454.27019.qmail@sources.redhat.com/T/

That is because the C99 specification should consider the flexible array
members to start at the end of a structure, but compilers prefer to put
padding at the end. A suggestion was made to allow compilers to allocate
padding after the VLA like compilers already did:

http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/n983.htm

However, upon thinking about it, whether or not we allocate end of
structure padding does not matter, so using offsetof() to calculate the
size of the structure is fine, so long as we do not mix it with sizeof()
on structures with no array members.

In the case that we mix them and padding causes offsetof(struct_t,
vla_member[0]) to differ from sizeof(struct_t), we would be doing unsafe
operations if we underallocate via `offsetof()` and then overcopy via
sizeof().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14372

module/lua/lfunc.h

@@ -13,10 +13,10 @@
 
 
 #define sizeCclosure(n)	(cast(int, sizeof(CClosure)) + \
-                         cast(int, sizeof(TValue)*((n)-1)))
+                         cast(int, sizeof(TValue)*((n))))
 
 #define sizeLclosure(n)	(cast(int, sizeof(LClosure)) + \
-                         cast(int, sizeof(TValue *)*((n)-1)))
+                         cast(int, sizeof(TValue *)*((n))))
 
 
 LUAI_FUNC Proto *luaF_newproto (lua_State *L);

module/lua/lobject.h

@@ -514,14 +514,14 @@ typedef struct UpVal {
 typedef struct CClosure {
   ClosureHeader;
   lua_CFunction f;
-  TValue upvalue[1];  /* list of upvalues */
+  TValue upvalue[];  /* list of upvalues */
 } CClosure;
 
 
 typedef struct LClosure {
   ClosureHeader;
   struct Proto *p;
-  UpVal *upvals[1];  /* list of upvalues */
+  UpVal *upvals[];  /* list of upvalues */
 } LClosure;
 
 

module/os/freebsd/zfs/zfs_debug.c

@@ -30,7 +30,7 @@ typedef struct zfs_dbgmsg {
 	list_node_t zdm_node;
 	time_t zdm_timestamp;
 	int zdm_size;
-	char zdm_msg[1]; /* variable length allocation */
+	char zdm_msg[];
 } zfs_dbgmsg_t;
 
 list_t zfs_dbgmsgs;
@@ -159,7 +159,7 @@ __zfs_dbgmsg(char *buf)
 
 	DTRACE_PROBE1(zfs__dbgmsg, char *, buf);
 
-	size = sizeof (zfs_dbgmsg_t) + strlen(buf);
+	size = sizeof (zfs_dbgmsg_t) + strlen(buf) + 1;
 	zdm = kmem_zalloc(size, KM_SLEEP);
 	zdm->zdm_size = size;
 	zdm->zdm_timestamp = gethrestime_sec();

module/os/linux/zfs/zfs_debug.c

@@ -30,7 +30,7 @@ typedef struct zfs_dbgmsg {
 	procfs_list_node_t	zdm_node;
 	uint64_t		zdm_timestamp;
 	int			zdm_size;
-	char			zdm_msg[1]; /* variable length allocation */
+	char			zdm_msg[]; /* variable length allocation */
 } zfs_dbgmsg_t;
 
 procfs_list_t zfs_dbgmsgs;
@@ -134,7 +134,7 @@ __set_error(const char *file, const char *func, int line, int err)
 void
 __zfs_dbgmsg(char *buf)
 {
-	int size = sizeof (zfs_dbgmsg_t) + strlen(buf);
+	int size = sizeof (zfs_dbgmsg_t) + strlen(buf) + 1;
 	zfs_dbgmsg_t *zdm = kmem_zalloc(size, KM_SLEEP);
 	zdm->zdm_size = size;
 	zdm->zdm_timestamp = gethrestime_sec();

module/zfs/vdev_indirect.c

@@ -270,7 +270,7 @@ typedef struct indirect_split {
 	 */
 	indirect_child_t *is_good_child;
 
-	indirect_child_t is_child[1]; /* variable-length */
+	indirect_child_t is_child[];
 } indirect_split_t;
 
 /*