xfs: remove the unused XFS_QMOPT_DQLOCK flag

The XFS_QMOPT_DQLOCK flag introduces major complexity in the quota subsystem
but isn't actually used anywhere.  So remove it and all the hazzles it
introduces.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>