ipv6: fix endianness error in icmpv6_err

IPv6 ping socket error handler doesn't correctly convert the new 32 bit
mtu to host endianness before using.

Cc: Lorenzo Colitti <lorenzo@google.com>
Fixes: 6d0bfe22 ("net: ipv6: Add IPv6 support to the ping socket.")
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>