checkpatch: ensure kconfig help checks only apply when we are adding help

When checking the length of the help we need to be sure we are seeing the
whole story before erroring.  Firstly we only want to check when adding
the help in the first place.  Second we need to be sure that we are seeing
the end of the entry, nominally when there is no context below or that
context shows the start of the next entry.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>