Btrfs: check return value of btrfs_search_slot properly

Doing an audit of where we use btrfs_search_slot only showed one place where we
don't check the return value of btrfs_search_slot properly.  Just fix
mark_extent_written to see if btrfs_search_slot failed and act accordingly.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/file.c

@@ -608,6 +608,8 @@ again:
 	key.offset = split;
 
 	ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
+	if (ret < 0)
+		goto out;
 	if (ret > 0 && path->slots[0] > 0)
 		path->slots[0]--;