fs/mpage: Use the new blk_opf_t type

Improve static type checking by using the new blk_opf_t type for the
combination of a block layer request with block layer request flags.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-50-bvanassche@acm.org


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