block/brd: Use the enum req_op type

Improve static type checking by using the enum req_op type for a
function argument that represents a request operation.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-13-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -256,7 +256,7 @@ static void copy_from_brd(void *dst, struct brd_device *brd,
  * Process a single bvec of a bio.
  */
 static int brd_do_bvec(struct brd_device *brd, struct page *page,
-			unsigned int len, unsigned int off, unsigned int op,
+			unsigned int len, unsigned int off, enum req_op op,
 			sector_t sector)
 {
 	void *mem;