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: NJianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Link: http://lkml.kernel.org/r/20200615010123.15596-1-jay.xu@rock-chips.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>