net: more accurate parsing of IPv4 header on IPConn

As shown in #9395, inaccurate implementation would be a cause of parsing
IPv4 header twice and corrupted upper-layer message issues.

Change-Id: Ia1a042e7ca58ee4fcb38fe9ec753c2ab100592ca
Reviewed-on: https://go-review.googlesource.com/3001


Reviewed-by: Ian Lance Taylor <iant@golang.org>