xfrm6: handling fragment

RFC4301 Section 7.1 says:

"7.1.  Tunnel Mode SAs that Carry Initial and Non-Initial Fragments

     All implementations MUST support tunnel mode SAs that are configured
     to pass traffic without regard to port field (or ICMP type/code or
     Mobility Header type) values.  If the SA will carry traffic for
     specified protocols, the selector set for the SA MUST specify the
     port fields (or ICMP type/code or Mobility Header type) as ANY.  An
     SA defined in this fashion will carry all traffic including initial
     and non-initial fragments for the indicated Local/Remote addresses
     and specified Next Layer protocol(s)."

But for IPv6, fragment is treated as a protocol.  This change catches
protocol transported in fragmented packet.  In IPv4, there is no
problem.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>