bpf: add bpf_probe_read_str into bpf_helpers.h

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3UHF8
CVE: NA

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Then, we can use bpf_probe_read_str() into ebpf program.
Signed-off-by: NYufen Yu <yuyufen@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

tools/testing/selftests/bpf/bpf_helpers.h

@@ -18,6 +18,8 @@ static int (*bpf_map_delete_elem)(void *map, void *key) =
 	(void *) BPF_FUNC_map_delete_elem;
 static int (*bpf_probe_read)(void *dst, int size, void *unsafe_ptr) =
 	(void *) BPF_FUNC_probe_read;
+static int (*bpf_probe_read_str)(void *dst, int size, void *unsafe_ptr) =
+	(void *) BPF_FUNC_probe_read_str;
 static unsigned long long (*bpf_ktime_get_ns)(void) =
 	(void *) BPF_FUNC_ktime_get_ns;
 static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =