block: Make bio_iov_bvec_set() accept pointer to const iov_iter

Make bio_iov_bvec_set() accept a pointer to const iov_iter, which means
that we can drop the undesirable casting to struct iov_iter pointer in
blk_rq_map_user_bvec().

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241202115727.2320401-1-john.g.garry@oracle.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>