[trusty SRU] LP#1404558: ipv6: fix swapped ipv4/ipv6 mtu_reduced callbacks

[Andy Whitcroft]

We have had reports of panics on machines with native IPv6 connectivity
in trusty since 3.13.0-43, downgrades to -40 fix the issue.  This has
been traced to a broken backport.  Following this email is a patch
sorting out the backport.

This patch has been tested by the reporter for 48 hours without further
incident, previously his machines had not makde 24 hours uptime with
these kernels.

Proposing for SRU to trusty.

-apw