fs/direct-io: Reduce the size of struct dio

fs/direct-io: Reduce the size of struct dio

Reduce the size of struct dio by combining the 'op' and 'op_flags' into
the new 'opf' member. Use the new blk_opf_t type to improve static type
checking. This patch does not change any functionality.

Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-49-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>