net: Refactor IP, UPD, and ICMP header writing functions

ICMP (ping) was reimplementing IP header code... it now shares code.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>