[PATCH] checkpatch: Fix SPDX license check with --root=<path>

[Joe Perches]

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.

Signed-off-by: Joe Perches <joe@xxxxxxxxxxx>
Reported-by: Charlemagne Lasse <charlemagnelasse@xxxxxxxxx>
Tested-by: Charlemagne Lasse <charlemagnelasse@xxxxxxxxx>
---
 scripts/checkpatch.pl | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index f25f708cd2a7..ce72cc4784e6 100755
--- a/scripts/checkpatch.pl
+++ b/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;
 }