Btrfs: send, don't error in the presence of subvols/snapshots

If we are doing an incremental send and the base snapshot has a
directory with name X that doesn't exist anymore in the second
snapshot and a new subvolume/snapshot exists in the second snapshot
that has the same name as the directory (name X), the incremental
send would fail with -ENOENT error. This is because it attempts
to lookup for an inode with a number matching the objectid of a
root, which doesn't exist.

Steps to reproduce:

    mkfs.btrfs -f /dev/sdd
    mount /dev/sdd /mnt

    mkdir /mnt/testdir
    btrfs subvolume snapshot -r /mnt /mnt/mysnap1

    rmdir /mnt/testdir
    btrfs subvolume create /mnt/testdir
    btrfs subvolume snapshot -r /mnt /mnt/mysnap2

    btrfs send -p /mnt/mysnap1 /mnt/mysnap2 -f /tmp/send.data

A test case for xfstests follows.
Reported-by: Robert White <rwhite@pobox.com>
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/send.c

@@ -1628,6 +1628,10 @@ static int lookup_dir_item_inode(struct btrfs_root *root,
 		goto out;
 	}
 	btrfs_dir_item_key_to_cpu(path->nodes[0], di, &key);
+	if (key.type == BTRFS_ROOT_ITEM_KEY) {
+		ret = -ENOENT;
+		goto out;
+	}
 	*found_inode = key.objectid;
 	*found_type = btrfs_dir_type(path->nodes[0], di);