media: Fix valgrind warnings

valgrind has no way to know that the kernel will fill links and pads
arrays passed to an ioctl through __user pointers. Allocate the arrays
with calloc() to make sure they get initialized. This also fixes the
potential integer overflow.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>