selftests/net: correct the return value for run_afpackettests

The run_afpackettests will be marked as passed regardless the return
value of those sub-tests in the script:
    --------------------
    running psock_tpacket test
    --------------------
    [FAIL]
    selftests: run_afpackettests [PASS]

Fix this by changing the return value for each tests.

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