block: Fix the bio.bi_opf comment

Commit ef295ec modified the Linux kernel such that the bottom bits
of the bi_opf member contain the operation instead of the topmost bits.
That commit did not update the comment next to bi_opf. Hence this patch.

From commit ef295ec:
-#define bio_op(bio)    ((bio)->bi_opf >> BIO_OP_SHIFT)
+#define bio_op(bio)    ((bio)->bi_opf & REQ_OP_MASK)

Cc: Christoph Hellwig <[email protected]>
Cc: Ming Lei <[email protected]>
Fixes: ef295ec ("block: better op and flags encoding")
Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

include/linux/blk_types.h

@@ -246,9 +246,8 @@ typedef unsigned int blk_qc_t;
 struct bio {
 	struct bio		*bi_next;	/* request queue link */
 	struct block_device	*bi_bdev;
-	unsigned int		bi_opf;		/* bottom bits req flags,
-						 * top bits REQ_OP. Use
-						 * accessors.
+	unsigned int		bi_opf;		/* bottom bits REQ_OP, top bits
+						 * req_flags.
 						 */
 	unsigned short		bi_flags;	/* BIO_* below */
 	unsigned short		bi_ioprio;