zonefs: use ZONEFS_SUPER_SIZE instead of PAGE_SIZE

zonefs: use ZONEFS_SUPER_SIZE instead of PAGE_SIZE

Use ZONEFS_SUPER_SIZE constant instead of PAGE_SIZE allocating memory for
reading the super block in zonefs_read_super().

While PAGE_SIZE technically isn't incorrect as Linux doesn't support pages
smaller than 4k ZONEFS_SUPER_SIZE is semantically more correct.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>