ceph: avoid divide by zero in __validate_layout()

If "l->stripe_unit" is zero the the mod on the next line will cause a
divide by zero bug.  This comes from the copy_from_user() in
ceph_ioctl_set_layout_policy().  Passing 0 is valid, though (it means
"do not change") so avoid the % check in that case.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>

fs/ceph/ioctl.c

@@ -42,7 +42,8 @@ static long __validate_layout(struct ceph_mds_client *mdsc,
 	/* validate striping parameters */
 	if ((l->object_size & ~PAGE_MASK) ||
 	    (l->stripe_unit & ~PAGE_MASK) ||
-	    ((unsigned)l->object_size % (unsigned)l->stripe_unit))
+	    (l->stripe_unit != 0 &&
+	     ((unsigned)l->object_size % (unsigned)l->stripe_unit)))
 		return -EINVAL;
 
 	/* make sure it's a valid data pool */