BACKPORT: mm: re-allow pinning of zero pfns

The commit referenced below subtly and inadvertently changed the logic to
disallow pinning of zero pfns.  This breaks device assignment with vfio
and potentially various other users of gup.  Exclude the zero page test
from the negation.

Link: https://lkml.kernel.org/r/165490039431.944052.12458624139225785964.stgit@omen


Fixes: 1c563432 ("mm: fix is_pinnable_page against a cma page")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reported-by: Yishai Hadas <yishaih@nvidia.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: John Dias <joaodias@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Zhangfei Gao <zhangfei.gao@linaro.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

(cherry picked from commit 034e5afa)

Bug: 255454074
Change-Id: I3ade7555a64fdf553bc08eb344b18b2f3de7aa14
Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com>