btrfs: incremental send after removing a directory

btrfs: incremental send after removing a directory

Test that, under a particular scenario, an incremental send
operation does not leak memory (which used to emit a warning in
dmesg/syslog).

This is a regression test for a btrfs kernel fix that has the title:
"Btrfs: send, fix warning due to late freeing of orphan_dir_info
structures".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>