btrfs: defrag: change BUG_ON to assertion in btrfs_defrag_leaves()

BugLink: https://bugs.launchpad.net/bugs/2084005

[ Upstream commit 51d4be540054be32d7ce28b63ea9b84ac6ff1db2 ]

The BUG_ON verifies a condition that should be guaranteed by the correct
use of the path search (with keep_locks and lowest_level set), an
assertion is the suitable check.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

fs/btrfs/defrag.c

@@ -521,7 +521,7 @@ static int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 	 * keep_locks set and lowest_level is 1, regardless of the value of
 	 * path->slots[1].
 	 */
-	BUG_ON(path->locks[1] == 0);
+	ASSERT(path->locks[1] != 0);
 	ret = btrfs_realloc_node(trans, root,
 				 path->nodes[1], 0,
 				 &last_ret,