scripts/documentation-file-ref-check: improve tools ref handling

There's a false positive on perf/util:

	tools/perf/util/s390-cpumsf.c: Documentation/perf.data-file-format.txt

The file is there at tools/perf/Documentation/, but the logic
with detects relative documentation references inside tools is
not capable of detecting it.

So, improve it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/documentation-file-ref-check

@@ -127,7 +127,7 @@ while (<IN>) {
 		if ($f =~ m/tools/) {
 			my $path = $f;
 			$path =~ s,(.*)/.*,$1,;
-			next if (grep -e, glob("$path/$ref $path/$fulref"));
+			next if (grep -e, glob("$path/$ref $path/../$ref $path/$fulref"));
 		}
 
 		# Discard known false-positives