quota: make Q_XQUOTASYNC a noop

Now that XFS takes quota reservations into account there is no need to flush
anything before reporting quotas - in addition to beeing fully transactional
all quota information is also 100% coherent with the rest of the filesystem
now.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>