xfs: move the log ioend workqueue to struct xlog

Move the workqueue used for log I/O completions from struct xfs_mount
to struct xlog to keep it self contained in the log code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: destroy the log workqueue after ensuring log ios are done]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>