Btrfs: skip locking if searching the commit root in csum lookup

It's not enough to just search the commit root, since we could be cow'ing the
very block we need to search through, which would mean that its locked and we'll
still deadlock.  So use path->skip_locking as well.  Thanks,
Signed-off-by: NJosef Bacik <josef@redhat.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/file-item.c

@@ -183,8 +183,10 @@ static int __btrfs_lookup_bio_sums(struct btrfs_root *root,
 	 * read from the commit root and sidestep a nasty deadlock
 	 * between reading the free space cache and updating the csum tree.
 	 */
-	if (btrfs_is_free_space_inode(root, inode))
+	if (btrfs_is_free_space_inode(root, inode)) {
 		path->search_commit_root = 1;
+		path->skip_locking = 1;
+	}
 
 	disk_bytenr = (u64)bio->bi_sector << 9;
 	if (dio)