kselftest/arm64: mte: Report filename on failing temp file creation

The MTE selftests create temporary files in /dev/shm, for later mmap-ing
them. When there is no tmpfs mounted on /dev/shm, or /dev/shm does not
exist in the first place (on minimal filesystems), the error message is
not giving good hints:
    # FAIL: Unable to open temporary file
    # FAIL: memory allocation
    not ok 17 Check initial tags with private mapping, ...

Add a perror() call, that gives both the filename and the actual error
reason, so that users get a chance of correcting that.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-12-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/mte/mte_common_util.c

@@ -337,6 +337,7 @@ int create_temp_file(void)
 	/* Create a file in the tmpfs filesystem */
 	fd = mkstemp(&filename[0]);
 	if (fd == -1) {
+		perror(filename);
 		ksft_print_msg("FAIL: Unable to open temporary file\n");
 		return 0;
 	}