quota: remove invalid optimization from quota_sync_all

Checking the "VFS" quota enabled and dirty bits from generic code means
this code will never get called for other implementations, e.g. XFS and
GFS2.  Grabbing the reference on the superblock really isn't much overhead
for a global Q_SYNC call, so just drop this optimization.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>