tools/testing/selftests/kcmp/kcmp_test.c: print reason for failure in kcmp_test

I was curious why sys_kcmp wasn't working, which led me to the testcase.
It turned out I hadn't enabled CHECKPOINT_RESTORE in the kernel I was
testing.  Add a decoding of errno to the testcase to make that obvious.
Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/kcmp/kcmp_test.c

@@ -72,7 +72,8 @@ int main(int argc, char **argv)
 		/* This one should return same fd */
 		ret = sys_kcmp(pid1, pid2, KCMP_FILE, fd1, fd1);
 		if (ret) {
-			printf("FAIL: 0 expected but %d returned\n", ret);
+			printf("FAIL: 0 expected but %d returned (%s)\n",
+				ret, strerror(errno));
 			ret = -1;
 		} else
 			printf("PASS: 0 returned as expected\n");
@@ -80,7 +81,8 @@ int main(int argc, char **argv)
 		/* Compare with self */
 		ret = sys_kcmp(pid1, pid1, KCMP_VM, 0, 0);
 		if (ret) {
-			printf("FAIL: 0 expected but %li returned\n", ret);
+			printf("FAIL: 0 expected but %li returned (%s)\n",
+				ret, strerror(errno));
 			ret = -1;
 		} else
 			printf("PASS: 0 returned as expected\n");