mm/cma.c: fix NULL pointer dereference when cma could not be activated

mainline inclusion
from mainline-v5.9-rc1
commit 835832ba
category: bugfix
bugzilla: 41397
CVE: NA

-------------------------------------------------

In some case the cma area could not be activated, but the cma_alloc be
used under this case, then the kernel will crash caused by NULL pointer
dereference.

Add bitmap valid check in cma_alloc to avoid this issue.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: http://lkml.kernel.org/r/20200615010123.15596-1-jay.xu@rock-chips.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>