block: move rq->start_time initialization to blk_rq_init()

rq->start_time was initialized in init_request_from_bio() so special
requests didn't have start_time set.  This has been okay as start_time
has been used only for fs requests; however, there is no indication of
this actually is the case or not.  Set rq->start_time in blk_rq_init()
and guarantee that all initialized rq's have its start_time set.  This
improves consistency at virtually no cost and future changes will make
use of the timestamp for !bio requests.

[ Impact: rq->start_time is valid for all requests ]

Signed-off-by: Tejun Heo <tj@kernel.org>