pktgen: Fix unsigned function that is returning negative vals

Every call to num_args() immediately checks the return value for
less than zero, as it will return -EFAULT for a failed get_user()
call.  So it makes no sense for the function to be declared as an
unsigned long.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>