xfs: don't lose solo dquot update transactions

Quota counter updates are tracked via incore objects which hang off the
xfs_trans object.  These changes are then turned into dirty log items in
xfs_trans_apply_dquot_deltas just prior to commiting the log items to
the CIL.

However, updating the incore deltas do not cause XFS_TRANS_DIRTY to be
set on the transaction.  In other words, a pure quota counter update
will be silently discarded if there are no other dirty log items
attached to the transaction.

This is currently not the case anywhere in the filesystem because quota
updates always dirty at least one other metadata item, but a subsequent
bug fix will add dquot log item precommits, so we actually need a dirty
dquot log item prior to xfs_trans_run_precommits.  Also let's not leave
a logic bomb.

Cc: <stable@vger.kernel.org> # v2.6.35
Fixes: 0924378a ("xfs: split out iclog writing from xfs_trans_commit()")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>