btrfs: add test for send issuing duplicated rmdir ops

btrfs: add test for send issuing duplicated rmdir ops

Regression test for btrfs incremental send issue where an rmdir
instruction was sent multiple times for the same target directory.
The number of times depended on the number of hardlinks against
the same inode inside the target directory. That inode must have
had the highest number of all the inodes that were children of the
directory. This made the btrfs receive command fail immediately once
it received the second rmdir instruction.

This issue is fixed by the following linux kernel btrfs patch:

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

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>