util/lint/checkpatch.pl: Untaint filename

This fixes the warning that is seen on the jenkins server:
Insecure dependency in piped open while running setgid at
util/lint/checkpatch.pl line 958.

Change-Id: I476efa76ef6a275584a47ec0ecf2315948d53e9d
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/23473
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

util/lint/checkpatch.pl

@@ -951,6 +951,10 @@ if ($git) {
 my $vname;
 for my $filename (@ARGV) {
 	my $FILE;
+
+	# coreboot: Mark filename as untainted
+	$filename =~ /^(.*)$/s or die; $filename = $1;
+
 	if ($git) {
 		open($FILE, '-|', "git format-patch -M --stdout -1 $filename") ||
 			die "$P: $filename: git format-patch failed - $!\n";