hfsplus: always use hfsplus_sync_fs to write the volume header

Remove opencoded writing of the volume header in hfsplus_fill_super
and hfsplus_put_super and offload it to hfsplus_sync_fs.  In the
put_super case this means we only write the superblock once instead
of twice.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>