Merge branch 'selftests-pmtu-Add-further-vti-vti6-MTU-and-PMTU-tests'

Stefano Brivio says:

====================
selftests: pmtu: Add further vti/vti6 MTU and PMTU tests

Patches 5/10 to 10/10 add tests to verify default MTU assignment
for vti4 and vti6 interfaces, to check that MTU values set on new
link and link changes are properly taken and validated, and to
verify PMTU exceptions on vti4 interfaces.

Patch 1/10 reverses function return codes as suggested by David
Ahern.

Patch 2/10 fixes the helper to fetch exceptions MTU to run in the
passed namespace.

Patches 3/10 and 4/10 are preparation work to make it easier to
introduce those tests.

v2: Reverse return codes, and make output prettier in 4/9 by
    using padded printf, test descriptions and buffered error
    strings. Remove accidental output to /dev/kmsg from 10/10
    (was 9/9).
====================
Signed-off-by: David S. Miller <davem@davemloft.net>