xfs: don't lose solo superblock counter update transactions

Superblock counter updates are tracked via per-transaction counters in
the xfs_trans object.  These changes are then turned into dirty log
items in xfs_trans_apply_sb_deltas just prior to commiting the log items
to the CIL.

However, updating the per-transaction counter deltas do not cause
XFS_TRANS_DIRTY to be set on the transaction.  In other words, a pure sb
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 sb
counter updates always dirty at least one other metadata item, but let's
not leave a logic bomb.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>