ICMPv6 error messages are tracked by extracting the conntrack tuple of
the inner packet and looking up the corresponding conntrack entry. Tuple
extraction uses the ->get_l4proto() callback, which in case of fragments
returns NEXTHDR_FRAGMENT instead of the upper protocol, even for the
first fragment when the entire next header is present, resulting in a
failure to find the correct connection tracking entry.

This patch changes ipv6_get_l4proto() to use ipv6_skip_exthdr() instead
of nf_ct_ipv6_skip_exthdr() in order to skip fragment headers when the
fragment offset is zero.

Signed-off-by: Patrick McHardy <kaber@trash.net>