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 <[e-mail]>
Cc: Ming Lei <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/brd.c

···
  * 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;