block: move blk_stat_add() to __blk_mq_end_request()

We want this next to blk_account_io_done() for the next change so that
we can call ktime_get() only once for both.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>