Add missing NULL in zpl_xattr_handlers

The xattr_resolve_name() helper function expects the registered
list of xattr handlers to be NULL terminated.  This NULL was
accidentally missing which could result in a NULL dereference.

Interestingly this issue only manifested itself on certain 32-bit
systems.  Presumably on 64-bit kernels we just always happen to
get lucky and the memory following the structure is zeroed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #594

module/zfs/zpl_xattr.c

@@ -674,4 +674,5 @@ xattr_handler_t *zpl_xattr_handlers[] = {
 	&zpl_xattr_acl_access_handler,
 	&zpl_xattr_acl_default_handler,
 #endif /* HAVE_POSIX_ACLS */
+	NULL
 };