btrfs: send: fix old buffer length in fs_path_ensure_buf

In "btrfs: send: lower memory requirements in common case" the code to
save the old_buf_len was incorrectly moved to a wrong place and broke
the original logic.
Reported-by: Filipe David Manana <fdmanana@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Filipe David Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/send.c

@@ -346,6 +346,9 @@ static int fs_path_ensure_buf(struct fs_path *p, int len)
 	if (p->buf_len >= len)
 		return 0;
 
+	path_len = p->end - p->start;
+	old_buf_len = p->buf_len;
+
 	/*
 	 * First time the inline_buf does not suffice
 	 */
@@ -368,9 +371,6 @@ static int fs_path_ensure_buf(struct fs_path *p, int len)
 		p->buf_len = ksize(p->buf);
 	}
 
-	path_len = p->end - p->start;
-	old_buf_len = p->buf_len;
-
 	if (p->reversed) {
 		tmp_buf = p->buf + old_buf_len - path_len - 1;
 		p->end = p->buf + p->buf_len - 1;