vfio: simplify kmalloc+copy_from_user to memdup_user

Generated by: coccinelle/api/memdup_user.cocci

Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>