tools/bpf: add ksym_get_addr() in trace_helpers

Given a kernel function name, ksym_get_addr() will return the kernel
address for this function, or 0 if it cannot find this function name
in /proc/kallsyms. This function will be used later when a kernel
address is used to initiate a kprobe perf event.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/trace_helpers.h

@@ -11,6 +11,7 @@ struct ksym {
 
 int load_kallsyms(void);
 struct ksym *ksym_search(long key);
+long ksym_get_addr(const char *name);
 
 typedef enum bpf_perf_event_ret (*perf_event_print_fn)(void *data, int size);