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: NAlex Williamson <alex.williamson@redhat.com>
Acked-by: NMinchan Kim <minchan@kernel.org>
Acked-by: NDavid Hildenbrand <david@redhat.com>
Reported-by: NYishai 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: NAndrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1600,7 +1600,7 @@ static inline bool is_pinnable_page(struct page *page)
 	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
 		return false;
 #endif
-	return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
+	return !is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page));
 }
 #else
 static inline bool is_pinnable_page(struct page *page)