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

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 */