net: Refactor packet length computations

Save the length when it is computed instead of forgetting it and
subtracting pointers to figure it out again.

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