checkpatch: add spell checking of email subject line

Only commit log and patch additions are checked for typos and spelling
errors currently.  Add a check of the email subject line too.
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2303,7 +2303,8 @@ sub process {
 		}
 
 # Check for various typo / spelling mistakes
-		if (defined($misspellings) && ($in_commit_log || $line =~ /^\+/)) {
+		if (defined($misspellings) &&
+		    ($in_commit_log || $line =~ /^(?:\+|Subject:)/i)) {
 			while ($rawline =~ /(?:^|[^a-z@])($misspellings)(?:$|[^a-z@])/gi) {
 				my $typo = $1;
 				my $typo_fix = $spelling_fix{lc($typo)};