selftests/x86/vdso: Fix no-vDSO segfaults

test_vdso would try to call a NULL pointer if the vDSO was missing.

vdso_restorer_32 hit a genuine failure: trying to use the
kernel-provided signal restorer doesn't work if the vDSO is missing.
Skip the test if the vDSO is missing, since the test adds no particular
value in that case.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/618ea7b8c55b10d08b1cb139e9a3a957934b8647.1584653439.git.luto@kernel.org

tools/testing/selftests/x86/test_vdso.c

@@ -259,6 +259,11 @@ static void test_one_clock_gettime(int clock, const char *name)
 
 static void test_clock_gettime(void)
 {
+	if (!vdso_clock_gettime) {
+		printf("[SKIP]\tNo vDSO, so skipping clock_gettime() tests\n");
+		return;
+	}
+
 	for (int clock = 0; clock < sizeof(clocknames) / sizeof(clocknames[0]);
 	     clock++) {
 		test_one_clock_gettime(clock, clocknames[clock]);