Take into account IPv6 fragment extension header when
calculating data size for each fragment.
Fixes: 7a838c8798a9 ("ip_frag: fix IPv6 when MTU sizes not aligned to 8 bytes")
Fixes: 0aa31d7a5929 ("ip_frag: add IPv6 fragmentation support")
Cc: stable@dpdk.org
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>