checkpatch: fix SPDX license check with --root=<path>

checkpatch uses the in-kernel script spdxcheck.py to validate the specific
license in a file or script.

This check can currently fail for a couple reasons:

o spdxcheck.py assumes the existence of git tree that may not
  exist for a bare source tree from something like a tarball
o the spdxcheck.py must be run from the top level root directory

So add a git existence test and set the subprocess subdirectory.

Link: http://lkml.kernel.org/r/2b32864324ae9c92948b002ec4c0c22409ed98f1.camel@perches.comSigned-off-by: NJoe Perches <joe@perches.com>
Reported-by: NCharlemagne Lasse <charlemagnelasse@gmail.com>
Tested-by: NCharlemagne Lasse <charlemagnelasse@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -852,9 +852,10 @@ sub is_maintained_obsolete {
 sub is_SPDX_License_valid {
 	my ($license) = @_;
 
-	return 1 if (!$tree || which("python") eq "" || !(-e "$root/scripts/spdxcheck.py"));
+	return 1 if (!$tree || which("python") eq "" || !(-e "$root/scripts/spdxcheck.py") || !(-e "$root/.git"));
 
-	my $status = `echo "$license" | python $root/scripts/spdxcheck.py -`;
+	my $root_path = abs_path($root);
+	my $status = `cd "$root_path"; echo "$license" | python scripts/spdxcheck.py -`;
 	return 0 if ($status ne "");
 	return 1;
 }