btrfs: test incremental send after deduplication on both snapshots

btrfs: test incremental send after deduplication on both snapshots

Test that an incremental send operation works after deduplicating into the
same file in both the parent and send snapshots.

This currently fails on btrfs and a kernel patch to fix it was submitted
with the subject:

  Btrfs: fix incremental send failure after deduplication

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