Merge branch 'pmtu-esp'

Vadim Fedorenko ays:

====================
Fix PMTU for ESP-in-UDP encapsulation

Bug 213669 uncovered regression in PMTU discovery for UDP-encapsulated
routes and some incorrect usage in udp tunnel fields. This series fixes
problems and also adds such case for selftests

v3:
 - update checking logic to account SCTP use case
v2:
 - remove refactor code that was in first patch
 - move checking logic to __udp{4,6}_lib_err_encap
 - add more tests, especially routed configuration
====================

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