xfs: remove leftover CoW reservations when remounting ro

When we're remounting the filesystem readonly, remove all CoW
preallocations prior to going ro.  If the fs goes down after the ro
remount, we never clean up the staging extents, which means xfs_check
will trip over them on a subsequent run.  Practically speaking, the next
mount will clean them up too, so this is unlikely to be seen.  Since we
shut down the cowblocks cleaner on remount-ro, we also have to make sure
we start it back up if/when we remount-rw.

Found by adding clonerange to fsstress and running xfs/017.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>