[PKTGEN]: don't use __constant_htonl()

The existing htonl() macro is smart enough to do the same code as
using __constant_htonl() and it looks cleaner.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>