[NETFILTER]: ip6_tables: use correct nexthdr value in ipv6_find_hdr()

nexthdr is NEXTHDR_FRAGMENT, the nexthdr value from the fragment header
is hp->nexthdr.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>