selftests: net: return Kselftest Skip code for skipped tests

When net test is skipped because of unmet dependencies and/or unsupported
configuration, it returns 0 which is treated as a pass by the Kselftest
framework. This leads to false positive result even when the test could
not be run.

Change it to return kselftest skip code when a test gets skipped to
clearly report that the test could not be run.

Kselftest framework SKIP code is 4 and the framework prints appropriate
messages to indicate that the test is skipped.

Change psock_tpacket to use ksft_exit_skip() when a non-root user runs
the test and add an explicit check for root and a clear message, instead
of failing the test when /sys/power/state file open fails.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>