Under certain situations, an incremental send operation can contain
a rmdir operation that will make the receiving end fail when attempting
to execute it, because the target directory is not yet empty.

Consider the following example:

  Parent snapshot:

  .                                                             (ino 256)
  |--- a/                                                       (ino 257)
  |    |--- c/                                                  (ino 260)
  |
  |--- del/                                                     (ino 259)
        |--- tmp/                                               (ino 258)
        |--- x/                                                 (ino 261)

  Send snapshot:

  .                                                             (ino 256)
  |--- a/                                                       (ino 257)
  |    |--- x/                                                  (ino 261)
  |
  |--- c/                                                       (ino 260)
       |--- tmp/                                                (ino 258)

1) When processing inode 258, we delay its rename operation because inode
   260 is its new parent in the send snapshot and it was not yet renamed
   (since 260 > 258, that is, beyond the current progress);

2) When processing inode 259, we realize we can not yet send an rmdir
   operation (against inode 259) because inode 258 was still not yet
   renamed/moved away from inode 259. Therefore we update data structures
   so that after inode 258 is renamed, we try again to see if we can
   finally send an rmdir operation for inode 259;

3) When we process inode 260, we send a rename operation for it followed
   by a rename operation for inode 258. Once we send the rename operation
   for inode 258 we then check if we can finally issue an rmdir for its
   previous parent, inode 259, by calling the can_rmdir() function with
   a value of sctx->cur_ino + 1 (260 + 1 = 261) for its "progress"
   argument. This makes can_rmdir() return true (value 1) because even
   though there's still a child inode of inode 259 that was not yet
   renamed/moved, which is inode 261, the given value of progress (261)
   is not lower then 261 (that is, not lower than the inode number of
   some child of inode 259). So we end up sending a rmdir operation for
   inode 259 before its child inode 261 is processed and renamed.

So fix this by passing the correct progress value to the call to
can_rmdir() from within apply_dir_move() (where we issue delayed rename
operations), which should match stcx->cur_ino (the number of the inode
currently being processed) and not sctx->cur_ino + 1.

A test case for fstests follows soon.
Signed-off-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
[Rewrote change log to be more detailed, clear and well formatted]
Signed-off-by: Filipe Manana <fdmanana@suse.com>