[PATCH] checkpatch: mute SOB warning in case SOB exists but without From header

[Dave Young]

checkpatch.pl report below warning but the SOB line is correct:
$ ./scripts/checkpatch.pl patches/test.patch
WARNING: Missing Signed-off-by: line by nominal patch author ''

Actually checkpatch.pl assumes every patch includes "From:" line,
this is true for a git formated patch, some saved mail format patch.

But the warning is boring for people who do not use git formated patch.
Change the script only print warning in case "From:" line exists.

Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
---
 scripts/checkpatch.pl |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-x86.orig/scripts/checkpatch.pl
+++ linux-x86/scripts/checkpatch.pl
@@ -2283,7 +2283,7 @@ sub process {
 	our $clean = 1;
 	my $signoff = 0;
 	my $author = '';
-	my $authorsignoff = 0;
+	my $authorsignoff = -1;
 	my $is_patch = 0;
 	my $is_binding_patch = -1;
 	my $in_header_lines = $file ? 0 : 1;
@@ -2609,6 +2609,8 @@ sub process {
 				$l =~ s/"//g;
 				if ($l =~ /^\s*signed-off-by:\s*\Q$author\E/i) {
 				    $authorsignoff = 1;
+				} else {
+				    $authorsignoff = 0;
 				}
 			}
 		}
@@ -6649,7 +6651,7 @@ sub process {
 		if ($signoff == 0) {
 			ERROR("MISSING_SIGN_OFF",
 			      "Missing Signed-off-by: line(s)\n");
-		} elsif (!$authorsignoff) {
+		} elsif ($authorsignoff == 0) {
 			WARN("NO_AUTHOR_SIGN_OFF",
 			     "Missing Signed-off-by: line by nominal patch author '$author'\n");
 		}