Merge branch 'net-ipv4-fixes-for-PMTU-when-link-MTU-changes'

Sabrina Dubroca says:

====================
net: ipv4: fixes for PMTU when link MTU changes

The first patch adapts the changes that commit e9fa1495 ("ipv6:
Reflect MTU changes on PMTU of exceptions for MTU-less routes") did in
IPv6 to IPv4: lower PMTU when the first hop's MTU drops below it, and
raise PMTU when the first hop was limiting PMTU discovery and its MTU
is increased.

The second patch fixes bugs introduced in commit d52e5a7e ("ipv4:
lock mtu in fnhe when received PMTU < net.ipv4.route.min_pmtu") that
only appear once the first patch is applied.

Selftests for these cases were introduced in net-next commit
e44e428f

 ("selftests: pmtu: add basic IPv4 and IPv6 PMTU tests")

v2: add cover letter, and fix a few small things in patch 1
====================

Signed-off-by: David S. Miller <davem@davemloft.net>