xfs: compute realtime device CoW staging extent reap limits dynamically

Calculate the maximum number of CoW staging extents that can be reaped
in a single transaction chain.  The rough calculation here is:

nr_extents = (logres - reservation used by any one step) /
		(space used by intents per extent)

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