btrfs: test incremental send/receive

btrfs: test incremental send/receive

Test for incremental send where the difference between the parent and
send snapshots is that for a subtree with the same path in both snapshots
(p1/p2), the root directories were swapped.

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

  Btrfs: incremental send, clear name from cache after orphanization

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Dave Chinner <david@fromorbit.com>