blk-mq: use blk_mq_rq_state() instead of testing REQ_ATOM_COMPLETE

blk_mq_check_inflight() and blk_mq_poll_hybrid_sleep() test
REQ_ATOM_COMPLETE to determine the request state.  Both uses are
speculative and we can test REQ_ATOM_STARTED and blk_mq_rq_state() for
equivalent results.  Replace the tests.  This will allow removing
REQ_ATOM_COMPLETE usages from blk-mq.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>