null_blk: Use blk_init_request_from_bio() instead of open-coding it

null_blk: Use blk_init_request_from_bio() instead of open-coding it

This patch changes the behavior of the null_blk driver for the
LightNVM mode as follows:
* REQ_FAILFAST_MASK is set for read-ahead requests.
* If no I/O priority has been set in the bio, the I/O priority is
  copied from the I/O context.
* The rq_disk member is initialized if bio->bi_bdev != NULL.
* req->errors is initialized to zero.

Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Matias Bjørling <[email protected]>
Cc: Adam Manzanares <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>