selftests/bpf: Add case to test bpf_in_interrupt kfunc

cd tools/testing/selftests/bpf
 ./test_progs -t irq
 #143/29  irq/in_interrupt:OK
 #143     irq:OK
 Summary: 1/34 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Leon Hwang <[email protected]>

Author: Asphaltt

tools/testing/selftests/bpf/progs/irq.c

@@ -563,4 +563,11 @@ int irq_wrong_kfunc_class_2(struct __sk_buff *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__success
+int in_interrupt(struct __sk_buff *ctx)
+{
+	return bpf_in_interrupt();
+}
+
 char _license[] SEC("license") = "GPL";