block: improve readability of blk_mq_end_request_batch()

It's faster and easier to read if we tolerate cur_hctx being NULL in
the "when to flush" condition. Rename last_hctx to cur_hctx while at it,
as it better describes the role of that variable.

Signed-off-by: Jens Axboe <axboe@kernel.dk>