selftests/bpf: Add test cases for various pointer specifiers

Extend snprintf negative tests to cover pointer specifiers to prevent
possible invalid handling of %p% from happening again.

 ./test_progs -t snprintf
 torvalds#302/1   snprintf/snprintf_positive:OK
 torvalds#302/2   snprintf/snprintf_negative:OK
 torvalds#302     snprintf:OK
 torvalds#303     snprintf_btf:OK
 Summary: 2/2 PASSED, 0 SKIPPED, 0 FAILED

Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>

Author: Ilya Shchipletsov

tools/testing/selftests/bpf/prog_tests/snprintf.c

@@ -116,6 +116,21 @@ static void test_snprintf_negative(void)
 	ASSERT_ERR(load_single_snprintf("%llc"), "invalid specifier 7");
 	ASSERT_ERR(load_single_snprintf("\x80"), "non ascii character");
 	ASSERT_ERR(load_single_snprintf("\x1"), "non printable character");
+
+	ASSERT_OK(load_single_snprintf("valid %p"), "valid usage");
+
+	ASSERT_ERR(load_single_snprintf("%p%"), "too many specifiers 1");
+	ASSERT_ERR(load_single_snprintf("%pK%"), "too many specifiers 2");
+	ASSERT_ERR(load_single_snprintf("%px%"), "too many specifiers 3");
+	ASSERT_ERR(load_single_snprintf("%ps%"), "too many specifiers 4");
+	ASSERT_ERR(load_single_snprintf("%pS%"), "too many specifiers 5");
+	ASSERT_ERR(load_single_snprintf("%pB%"), "too many specifiers 6");
+	ASSERT_ERR(load_single_snprintf("%pi4%"), "too many specifiers 7");
+	ASSERT_ERR(load_single_snprintf("%pI4%"), "too many specifiers 8");
+	ASSERT_ERR(load_single_snprintf("%pi6%"), "too many specifiers 9");
+	ASSERT_ERR(load_single_snprintf("%pI6%"), "too many specifiers 10");
+	ASSERT_ERR(load_single_snprintf("%pks%"), "too many specifiers 11");
+	ASSERT_ERR(load_single_snprintf("%pus%"), "too many specifiers 12");
 }
 
 void test_snprintf(void)