selftests/bpf: Add tests for bpf_strnstr

Add two tests for bpf_strnstr():

    bpf_strnstr("", "", 0) = 0
    bpf_strnstr("hello world", "hello", 5) = 0

Signed-off-by: Rong Tao <[email protected]>

Author: Rtoax

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

@@ -30,6 +30,8 @@ __test(2) int test_strcspn(void *ctx) { return bpf_strcspn(str, "lo"); }
 __test(6) int test_strstr_found(void *ctx) { return bpf_strstr(str, "world"); }
 __test(-ENOENT) int test_strstr_notfound(void *ctx) { return bpf_strstr(str, "hi"); }
 __test(0) int test_strstr_empty(void *ctx) { return bpf_strstr(str, ""); }
+__test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr("", "", 0); }
+__test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr(str, "hello", 5); }
 __test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr(str, "hello", 6); }
 __test(-ENOENT) int test_strnstr_notfound(void *ctx) { return bpf_strnstr(str, "hi", 10); }
 __test(0) int test_strnstr_empty(void *ctx) { return bpf_strnstr(str, "", 1); }