dma-pool: Fix an uninitialized variable bug in atomic_pool_expand()

The "page" pointer can be used with out being initialized.

Fixes: d7e673ec

 ("dma-pool: Only allocate from CMA when in same memory zone")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>