btrfs: add regression test for btrfs send with orphans

btrfs: add regression test for btrfs send with orphans

Regression test for a btrfs issue where we create a RO snapshot
to use for a send operation, which fails with a -ESTALE error,
due to the presence of orphan inodes accessible through the
snapshot's commit root but no longer present through the main
root.

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

  Btrfs: update commit root on snapshot creation after orphan cleanup

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>