btrfs: test send with snapshots that have files deleted while open

btrfs: test send with snapshots that have files deleted while open

Test that we are able to do send operations when one of the source
snapshots (or subvolume) has a file that is deleted while there is
still a open file descriptor for that file.

This test is motivated by a bug found in btrfs which is fixed by a patch
for the linux kernel titled:

  "Btrfs: fix send failure when root has deleted files still open"

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