block: move rq_ioc() to blk.h

block: move rq_ioc() to blk.h

We want to use it outside of blk-core.c.

Signed-off-by: Jens Axboe <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Omar Sandoval <[email protected]>