ipv6: Fix exthdrs offload registration.

Without this fix, ipv6_exthdrs_offload_init doesn't register IPPROTO_DSTOPTS
offload, but returns 0 (as the IPPROTO_ROUTING registration actually succeeds).

This then causes the ipv6_gso_segment to drop IPv6 packets with IPPROTO_DSTOPTS
header.

The issue detected and the fix verified by running MS HCK Offload LSO test on
top of QEMU Windows guests, as this test sends IPv6 packets with
IPPROTO_DSTOPTS.

Signed-off-by: Anton Nayshtut <anton@swortex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>