selftests: pmtu.sh: use $ksft_skip for skipped return code

This test uses return code 2 as a hard-coded skipped state, let's use
the kselftest framework skip code variable $ksft_skip instead to make
it more readable and easier to maintain.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>