drm/amdkfd: Integer overflows in ioctl

args->n_devices is a u32 that comes from the user.  The multiplication
could overflow on 32 bit systems possibly leading to privilege
escalation.

Fixes: 5ec7e028 ("drm/amdkfd: Add ioctls for GPUVM memory management")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>