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 <[e-mail]>
Acked-by: Shuah Khan <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/config

···
 CONFIG_NF_TABLES_IPV4=y
 CONFIG_NFT_CHAIN_NAT_IPV6=m
 CONFIG_NFT_CHAIN_NAT_IPV4=m
+CONFIG_KALLSYMS=y

tools/testing/selftests/net/run_afpackettests

···
 echo "--------------------"
 echo "running psock_tpacket test"
 echo "--------------------"
-./in_netns.sh ./psock_tpacket
-if [ $? -ne 0 ]; then
-	echo "[FAIL]"
-	ret=1
+if [ -f /proc/kallsyms ]; then
+	./in_netns.sh ./psock_tpacket
+	if [ $? -ne 0 ]; then
+		echo "[FAIL]"
+		ret=1
+	else
+		echo "[PASS]"
+	fi
 else
-	echo "[PASS]"
+	echo "[SKIP] CONFIG_KALLSYMS not enabled"
 fi
 
 echo "--------------------"