Btrfs: send, don't send rmdir for same target multiple times

When doing an incremental send, if we delete a directory that has N > 1
hardlinks for the same file and that file has the highest inode number
inside the directory contents, an incremental send would send N times an
rmdir operation against the directory. This made the btrfs receive command
fail on the second rmdir instruction, as the target directory didn't exist
anymore.

Steps to reproduce the issue:

    $ mkfs.btrfs -f /dev/sdb3
    $ mount /dev/sdb3 /mnt/btrfs
    $ mkdir -p /mnt/btrfs/a/b/c
    $ echo 'ola mundo' > /mnt/btrfs/a/b/c/foo.txt
    $ ln /mnt/btrfs/a/b/c/foo.txt /mnt/btrfs/a/b/c/bar.txt
    $ btrfs subvolume snapshot -r /mnt/btrfs /mnt/btrfs/snap1
    $ btrfs send /mnt/btrfs/snap1 -f /tmp/base.send
    $ rm -f /mnt/btrfs/a/b/c/foo.txt
    $ rm -f /mnt/btrfs/a/b/c/bar.txt
    $ rmdir /mnt/btrfs/a/b/c
    $ btrfs subvolume snapshot -r /mnt/btrfs /mnt/btrfs/snap2
    $ btrfs send -p /mnt/btrfs/snap1 /mnt/btrfs/snap2 -f /tmp/incremental.send

    $ umount /mnt/btrfs
    $ mkfs.btrfs -f /dev/sdb3
    $ mount /dev/sdb3 /mnt/btrfs
    $ btrfs receive /mnt/btrfs -f /tmp/base.send
    $ btrfs receive /mnt/btrfs -f /tmp/incremental.send

The second btrfs receive command failed with:

    ERROR: rmdir o259-6-0 failed. No such file or directory

A test case for xfstests follows.
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/send.c

@@ -3085,6 +3085,7 @@ static int process_recorded_refs(struct send_ctx *sctx, int *pending_move)
 	u64 ow_gen;
 	int did_overwrite = 0;
 	int is_orphan = 0;
+	u64 last_dir_ino_rm = 0;
 
 verbose_printk("btrfs: process_recorded_refs %llu\n", sctx->cur_ino);
 
@@ -3349,7 +3350,8 @@ verbose_printk("btrfs: process_recorded_refs %llu\n", sctx->cur_ino);
 			ret = send_utimes(sctx, cur->dir, cur->dir_gen);
 			if (ret < 0)
 				goto out;
-		} else if (ret == inode_state_did_delete) {
+		} else if (ret == inode_state_did_delete &&
+			   cur->dir != last_dir_ino_rm) {
 			ret = can_rmdir(sctx, cur->dir, sctx->cur_ino);
 			if (ret < 0)
 				goto out;
@@ -3361,6 +3363,7 @@ verbose_printk("btrfs: process_recorded_refs %llu\n", sctx->cur_ino);
 				ret = send_rmdir(sctx, valid_path);
 				if (ret < 0)
 					goto out;
+				last_dir_ino_rm = cur->dir;
 			}
 		}
 	}