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).

Stefano Brivio (10):
selftests: pmtu: Reverse return codes of functions
selftests: pmtu: Use namespace command prefix to fetch route mtu
selftests: pmtu: Factor out MTU parsing helper
selftests: pmtu: Introduce support for multiple tests
selftests: pmtu: Add pmtu_vti4_default_mtu test
selftests: pmtu: Add pmtu_vti6_default_mtu test
selftests: pmtu: Add test_pmtu_vti4_exception test
selftests: pmtu: Add pmtu_vti4_link_add_mtu test
selftests: pmtu: Add pmtu_vti6_link_add_mtu test
selftests: pmtu: Add pmtu_vti6_link_change_mtu test

 tools/testing/selftests/net/pmtu.sh | 442 ++++++++++++++++++++++++++++++------
 1 file changed, 376 insertions(+), 66 deletions(-)

2.15.1