checkpatch: warn on subject line not followed by blank line

Fixed case when no warning generated for long subject line that is
wrapped to more than one line, and all lines are less than line limit.

New warning message added:
"non-blank line after summary line"
Now there are two warnings possible for the subject line, the original
line over limit and the new one. Depending on the error(s) any
combination of the two warnings are possible.

Commit text requirements now:
1) Must be less than 75 characters
2) Must be followed by blank line.

Signed-off-by: Richard Adams <c_adams@quicinc.com>
(cherry picked from commit 78d0db4aa1212e00696e25b7daa9fc4ecd86f954)
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>