selftests/net: skip psock_tpacket test if KALLSYMS was not enabled

The psock_tpacket test will need to access /proc/kallsyms, this would
require the kernel config CONFIG_KALLSYMS to be enabled first.

Apart from adding CONFIG_KALLSYMS to the net/config file here, check the
file existence to determine if we can run this test will be helpful to
avoid a false-positive test result when testing it directly with the
following commad against a kernel that have CONFIG_KALLSYMS disabled:
    make -C tools/testing/selftests TARGETS=net run_tests

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>