mm/gup: Increment the page refcount before the pincount

We should always increase the refcount before doing anything else to
the page so that other page users see the elevated refcount first.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>