dm-bufio: virt_to_phys() doesn't change remainder modulo PAGE_SIZE

... so virt_to_phys(p) & (PAGE_SIZE - 1) is a very odd way to
spell offset_in_page(p).
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

drivers/md/dm-bufio.c

@@ -630,7 +630,7 @@ static void use_inline_bio(struct dm_buffer *b, int rw, sector_t block,
 	do {
 		if (!bio_add_page(&b->bio, virt_to_page(ptr),
 				  len < PAGE_SIZE ? len : PAGE_SIZE,
-				  virt_to_phys(ptr) & (PAGE_SIZE - 1))) {
+				  offset_in_page(ptr))) {
 			BUG_ON(b->c->block_size <= PAGE_SIZE);
 			use_dmio(b, rw, block, end_io);
 			return;