block: rename bio bi_rw to bi_opf

Since commit 63a4cc24867d, bio->bi_rw contains flags in the lower
portion and the op code in the higher portions. This means that
old code that relies on manually setting bi_rw is most likely
going to be broken. Instead of letting that brokeness linger,
rename the member, to force old and out-of-tree code to break
at compile time instead of at runtime.

No intended functional changes in this commit.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index 20bae50..571a6c7 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -128,7 +128,7 @@
 	struct pmem_device *pmem = q->queuedata;
 	struct nd_region *nd_region = to_region(pmem);
 
-	if (bio->bi_rw & REQ_FLUSH)
+	if (bio->bi_opf & REQ_FLUSH)
 		nvdimm_flush(nd_region);
 
 	do_acct = nd_iostat_start(bio, &start);
@@ -144,7 +144,7 @@
 	if (do_acct)
 		nd_iostat_end(bio, start);
 
-	if (bio->bi_rw & REQ_FUA)
+	if (bio->bi_opf & REQ_FUA)
 		nvdimm_flush(nd_region);
 
 	bio_endio(bio);