xfs: compute file mapping reap limits dynamically

Reaping file fork mappings is a little different -- log recovery can
free the blocks for us, so we only try to process a single mapping at a
time.  Therefore, we only need to figure out the maximum number of
blocks that we can invalidate in a single transaction.

The rough calculation here is:

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

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