numa, sched, mm: Fix NULL-ptr deref

numa, sched, mm: Fix NULL-ptr deref

Dan reported that there's a possible NULL pointer deref in this logic,
fix that. Further fix it to avoid a possible inf. loop (completely
unlikely in the case no vma is migratable). Also don't endlessly loop
on large length, simply truncate at the end of the address-space and
restart on the next go.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Link: http://lkml.kernel.org/n/tip-mnkio02xxtttiepsg9ek6qkw@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>