staging/cxt1e1/linux.c: Correct arbitrary memory write in c4_ioctl()

The function c4_ioctl() writes data from user in ifr->ifr_data
to the kernel struct data arg, without any iolen bounds checking.
This can lead to a arbitrary write outside of the struct data arg.
Corrected by adding bounds-checking of iolen before the copy_from_user().

Signed-off-by: Salva Peiró <speiro@ai2.upv.es>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>