selftests/mm/mlock: print error on failure

It's not really possible to start diagnosing this without knowing the
actual error.

Also update the mlock2 helper to behave like libc would by setting errno
and returning -1.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Brendan Jackman <[e-mail]>
Cc: Dev Jain <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: Mateusz Guzik <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/mlock-random-test.c

···
 				       MLOCK_ONFAULT);
 
 		if (ret)
-			ksft_exit_fail_msg("%s() failure at |%p(%d)| mlock:|%p(%d)|\n",
+			ksft_exit_fail_msg("%s() failure (%s) at |%p(%d)| mlock:|%p(%d)|\n",
 					   is_mlock ? "mlock" : "mlock2",
-					   p, alloc_size,
+					   strerror(errno), p, alloc_size,
 					   p + start_offset, lock_size);
 	}
 

tools/testing/selftests/mm/mlock2.h

···
 
 static int mlock2_(void *start, size_t len, int flags)
 {
-	return syscall(__NR_mlock2, start, len, flags);
+	int ret = syscall(__NR_mlock2, start, len, flags);
+
+	if (ret) {
+		errno = ret;
+		return -1;
+	}
+	return 0;
 }
 
 static FILE *seek_to_smaps_entry(unsigned long addr)