xfs: add helpers to compute transaction reservation for finishing intent items

In the transaction reservation code, hoist the logic that computes the
reservation needed to finish one log intent item into separate helper
functions.  These will be used in subsequent patches to estimate the
number of blocks that an online repair can commit to reaping in the same
transaction as the change committing the new data structure.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>