block: add bio_rw_flagged() for testing bio->bi_rw

The existing functions for checking bio->bi_rw are badly named. So lets
mirror what we do for bio->bi_flags testing, use a properly named
function so that it's immediately obvious what is being tested.

Maintain compatability names for the old macros, eventually we'll get
rid of these.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>