btrfs: add test for btrfs send issuing premature rmdir operations

btrfs: add test for btrfs send issuing premature rmdir operations

Regression test for btrfs incremental send issue where a rmdir instruction
is sent against an orphan directory inode which is not empty yet, causing
btrfs receive to fail when it attempts to remove the directory.

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

    Btrfs: fix send attempting to rmdir non-empty directories

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>