bcache: use bvec_virt

Use bvec_virt instead of open coding it.  Note that the existing code is
fine despite ignoring bv_offset as the bio is known to contain exactly
one page from the page allocator per bio_vec.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20210804095634.460779-10-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -378,7 +378,7 @@ static void do_btree_node_write(struct btree *b)
 		struct bvec_iter_all iter_all;
 
 		bio_for_each_segment_all(bv, b->bio, iter_all) {
-			memcpy(page_address(bv->bv_page), addr, PAGE_SIZE);
+			memcpy(bvec_virt(bv), addr, PAGE_SIZE);
 			addr += PAGE_SIZE;
 		}