Stable backport needed for "net: don't call update_pmtu unconditionally"

Hi,

f15ca723c1 ("net: don't call update_pmtu unconditionally") fixes NULL derefs
caused by a93bf0ff4490 ("vxlan: update skb dst pmtu on tx path"), which was
backported to 4.14.24.  We've received reports of panics with Open vSwitch
and Weave through 4.14.30 (https://github.com/coreos/bugs/issues/2382) and
at least the latter is resolved by backporting the patch.

Thanks,
--Benjamin Gilbert