ctdb-common: Fix aliasing issue in IPv6 checksum

ctdb-common: Fix aliasing issue in IPv6 checksum

Since commit 9c51b278b1700cd5f3e2addc19b7c711cc2ea10b the compiler has
been able to inline the affected call to uint16_checksum().  Given
that the data (phdr) is being accessed by an incompatible
pointer (data) there is an aliasing problem when the call is inlined.
This results in incorrect behaviour with -O2/-O3 when compiling with
at least GCC 6, 7, and 8.

Fix this by making the types compatible.

Also fixes CID 1437604 (Reliance on integer endianness).  This is a
false positive because the uint16_checksum doesn't depend on the order
of the input uint16_t items.

https://bugzilla.samba.org/show_bug.cgi?id=13588

Pair-programmed-with: Amitay Isaacs <amitay@gmail.com>
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>