zonefs: Call page_address() on page acquired with GFP_KERNEL flag

zonefs_read_super() acquires a page with alloc_page(GFP_KERNEL). That
page cannot come from ZONE_HIGHMEM, thus there's no need to map it with
kmap().

Therefore, use a plain page_address() on that page.

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>