blk-mq: ->timeout should be cleared in blk_mq_rq_ctx_init()

It'll be used in blk_mq_start_request() to set a potential timeout
for the request, so clear it to zero at alloc time to ensure that
we know if someone has set it or not.

Fixes random early timeouts on NVMe testing.

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