[IPV6]: MTU discovery check in ip6_fragment()

Adds a check in ip6_fragment() mirroring ip_fragment() for packets
that we can't fragment, and sends an ICMP Packet Too Big message
in response.

Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>