mm,kdb,kgdb: Add a debug reference for the kdb kmap usage

The kdb kmap should never get used outside of the kernel debugger
exception context.

Signed-off-by: Jason <Wessel&lt;jason.wessel@windriver.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Ingo Molnar <mingo@elte.hu>
CC: linux-mm@kvack.org