net: fix IPv6 checksum

net: fix IPv6 checksum

For rte_ipv6_phdr_cksum() gcc 4.8.* with "-O3" not always generates
correct code.
Sometimes it 'forgets' to put len and proto fields of psd_header on the stack.
To overcome that problem and speedup things a bit, refactored rte_raw_cksum()
by splitting ipv6 pseudo-header csum calculation into 3 phases:
1. calc sum for src & dst addresses
2. add sum for proto & len.
3. finalise sum
That makes gcc to generate valid code and helps to avoid any copying.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>