Add zfs_btree_verify_intensity kernel module parameter

I see a few issues in the issue tracker that might be aided by being
able to turn this on. We have no module parameter for it, so I would
like to add one.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13874

cmd/zdb/zdb.c

@@ -112,7 +112,7 @@ extern int zfs_vdev_async_read_max_active;
 extern boolean_t spa_load_verify_dryrun;
 extern boolean_t spa_mode_readable_spacemaps;
 extern int zfs_reconstruct_indirect_combinations_max;
-extern int zfs_btree_verify_intensity;
+extern uint_t zfs_btree_verify_intensity;
 
 static const char cmdname[] = "zdb";
 uint8_t dump_opt[256];

man/man4/zfs.4

@@ -1348,6 +1348,22 @@ _
 .TE
 .Sy \& * No Requires debug build.
 .
+.It Sy zfs_btree_verify_intensity Ns = Ns Sy 0 Pq uint
+Enables btree verification.
+The following settings are culminative:
+.TS
+box;
+lbz r l l .
+	Value	Description
+
+	1	Verify height.
+	2	Verify pointers from children to parent.
+	3	Verify element counts.
+	4	Verify element order. (expensive)
+*	5	Verify unused memory is poisoned. (expensive)
+.TE
+.Sy \& * No Requires debug build.
+.
 .It Sy zfs_free_leak_on_eio Ns = Ns Sy 0 Ns | Ns 1 Pq int
 If destroy encounters an
 .Sy EIO

module/zfs/btree.c

@@ -53,7 +53,7 @@ kmem_cache_t *zfs_btree_leaf_cache;
  * (while the asymptotic complexity of the other steps is the same, the
  * importance of the constant factors cannot be denied).
  */
-int zfs_btree_verify_intensity = 0;
+uint_t zfs_btree_verify_intensity = 0;
 
 /*
  * Convenience functions to silence warnings from memcpy/memmove's
@@ -2171,3 +2171,9 @@ zfs_btree_verify(zfs_btree_t *tree)
 		return;
 	zfs_btree_verify_poison(tree);
 }
+
+/* BEGIN CSTYLED */
+ZFS_MODULE_PARAM(zfs, zfs_, btree_verify_intensity, UINT, ZMOD_RW,
+	"Enable btree verification. Levels above 4 require ZFS be built "
+	"with debugging");
+/* END CSTYLED */