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 <[e-mail]>
Acked-by: Cyrill Gorcunov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/kcmp/kcmp_test.c

···
 		/* 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");
···
 		/* 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");