block: Document that bio_op() uses the data type of bio.bi_opf

Make it clear that the sizeof(unsigned int) expression in BIO_OP_SHIFT
refers to the bi_opf member of struct bio.
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Damien Le Moal <damien.lemoal@hgst.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk_types.h

@@ -89,7 +89,7 @@ struct bio {
 	struct bio_vec		bi_inline_vecs[0];
 };
 
-#define BIO_OP_SHIFT	(8 * sizeof(unsigned int) - REQ_OP_BITS)
+#define BIO_OP_SHIFT	(8 * FIELD_SIZEOF(struct bio, bi_opf) - REQ_OP_BITS)
 #define bio_op(bio)	((bio)->bi_opf >> BIO_OP_SHIFT)
 
 #define bio_set_op_attrs(bio, op, op_flags) do {		\