Btrfs: fix wrong search path initialization before searching tree root

To search tree root without transaction protection, we should neither search commit
root nor skip locking here, fix it.
Signed-off-by: NWang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/send.c

@@ -2095,7 +2095,7 @@ static int send_subvol_begin(struct send_ctx *sctx)
 	char *name = NULL;
 	int namelen;
 
-	path = alloc_path_for_send();
+	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;