selftests/net: correct the return value for run_netsocktests

The run_netsocktests will be marked as passed regardless the actual test
result from the ./socket:

    selftests: net: run_netsocktests
    ========================================
    --------------------
    running socket test
    --------------------
    [FAIL]
    ok 1..6 selftests: net: run_netsocktests [PASS]

This is because the test script itself has been successfully executed.
Fix this by exit 1 when the test failed.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>