dm: rename request variables to bios

Use 'bio' in the name of variables and functions that deal with
bios rather than 'request' to avoid confusion with the normal
block layer use of 'request'.

No functional changes.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-zero.c b/drivers/md/dm-zero.c
index 69a5c3b..c99003e 100644
--- a/drivers/md/dm-zero.c
+++ b/drivers/md/dm-zero.c
@@ -25,7 +25,7 @@
 	/*
 	 * Silently drop discards, avoiding -EOPNOTSUPP.
 	 */
-	ti->num_discard_requests = 1;
+	ti->num_discard_bios = 1;
 
 	return 0;
 }