ctdb-common: Avoid aliasing errors during code optimization

ctdb-common: Avoid aliasing errors during code optimization

When compiling with GCC 10.x and -O3 optimization, the IP checksum
calculation code generates wrong checksum.  The function uint16_checksum
gets inlined during optimization and ip4pkt->tcp data gets wrongly
aliased.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14537

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Wed Oct 21 05:52:28 UTC 2020 on sn-devel-184

(cherry picked from commit 6aa396b0cd1f83f45cb76a4f3123d99135e8dd8c)