net: dropreason: add SKB_DROP_REASON_IPV6_BAD_EXTHDR

This drop reason can be used whenever an IPv6 packet
has a malformed extension header.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>