checkpatch: add spell checking of email subject line

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>