checkpatch: add a multiple blank lines test

Multiple consecutive blank lines waste screen space.  Emit a --strict
only message with these blank lines.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>