selftests: net: add missing config for pmtu.sh tests

selftests: net: add missing config for pmtu.sh tests

The mentioned test uses a few Kconfig still missing the
net config, add them.

Before:
  # Error: Specified qdisc kind is unknown.
  # Error: Specified qdisc kind is unknown.
  # Error: Qdisc not classful.
  # We have an error talking to the kernel
  # Error: Qdisc not classful.
  # We have an error talking to the kernel
  #   policy_routing not supported
  # TEST: ICMPv4 with DSCP and ECN: PMTU exceptions                     [SKIP]

After:
  # TEST: ICMPv4 with DSCP and ECN: PMTU exceptions                     [ OK ]

Fixes: ec730c3e1f0e ("selftest: net: Test IPv4 PMTU exceptions with DSCP and ECN")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/8d27bf6762a5c7b3acc457d6e6872c533040f9c1.1706635101.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>