mm/hugetlb: avoid hardcoding while checking if cma is enabled

commit dbda8fea upstream.

hugetlb_cma[0] can be NULL due to various reasons, for example, node0
has no memory.  so NULL hugetlb_cma[0] doesn't necessarily mean cma is
not enabled.  gigantic pages might have been reserved on other nodes.
This patch fixes possible double reservation and CMA leak.

[akpm@linux-foundation.org: fix CONFIG_CMA=n warning]
[sfr@canb.auug.org.au: better checks before using hugetlb_cma]
  Link: http://lkml.kernel.org/r/20200721205716.6dbaa56b@canb.auug.org.au

Fixes: cf11e85f

 ("mm: hugetlb: optionally allocate gigantic hugepages using cma")
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200710005726.36068-1-song.bao.hua@hisilicon.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>