selftests/bpf: Fix possible ksyms test failure with LTO kernel

In my locally build clang LTO kernel (enabling CONFIG_LTO and
CONFIG_LTO_CLANG_THIN), ksyms test failed like:
  test_ksyms:PASS:kallsyms_fopen 0 nsec
  test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
  #118     ksyms:FAIL

The reason is that 'bpf_link_fops' is renamed to
  bpf_link_fops.llvm.8325593422554671469
Due to cross-file inlining, the static variable 'bpf_link_fops'
in syscall.c is used by a function in another file. To avoid
potential duplicated names, the llvm added suffix '.llvm.<hash>'.

To fix the failure, we can skip this test with LTO kernel
if the symbol 'bpf_link_fops' is not found in kallsyms.

After this patch, with the same LTO kernel:
  #118     ksyms:SKIP

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Author: Yonghong Song

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

@@ -21,7 +21,11 @@ void test_ksyms(void)
 		return;
 	}
 	if (err == -ENOENT) {
-		ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
+		/* bpf_link_fops might be renamed to bpf_link_fops.llvm.<hash> in LTO kernel. */
+		if (check_lto_kernel() == 1)
+			test__skip();
+		else
+			ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
 		return;
 	}