block: get rid of struct blk_issue_stat

struct blk_issue_stat squashes three things into one u64:

- The time the driver started working on a request
- The original size of the request (for the io.low controller)
- Flags for writeback throttling

It turns out that on x86_64, we have a 4 byte hole in struct request
which we can fill with the non-timestamp fields from blk_issue_stat,
simplifying things quite a bit.

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