ip_frag: fix IPv6 when MTU sizes not aligned to 8 bytes

ip_frag: fix IPv6 when MTU sizes not aligned to 8 bytes

The same issue was fixed on for the ipv4 version of this routine in
commit 8d4d3a4f7337 ("ip_frag: handle MTU sizes not aligned to 8 bytes").
Briefly, the size of an ipv6 header is always 40 bytes.  With an MTU of
1500, this will never produce a multiple of 8 bytes for the frag_size
and this routine can never succeed. Since RTE_ASSERTS are disabled by
default, this failure is typically ignored.

To fix this, round down to the nearest 8 bytes and use this when
producing the fragments.

Fixes: 0aa31d7a5929 ("ip_frag: add IPv6 fragmentation support")
Cc: stable@dpdk.org
Signed-off-by: Chas Williams <chas3@att.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>