btrfs: test send on subvolume with delalloc after setting it to RO mode

Test that if we have a subvolume/snapshot that is writable, has a file
with unflushed delalloc (buffered writes not yet flushed), turn the
subvolume to readonly mode and then use it for send a operation, the send
stream will contain the delalloc data - that is, no data loss happens.

This currently files on btrfs (data loss) but is fixed by a patch for
the linux kernel titled:

  "Btrfs: send, flush dellaloc in order to avoid data loss"

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