GFS2: Consolidate transaction blocks calculation for dir add

There are three cases where we need to calculate the number of
blocks to reserve in a transaction involving linking an inode
into a directory. The one in rename is a bit more complicated,
but the basis of it is the same as for link and create. So it
makes sense to move this calculation into a single function
rather than repeating it three times.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>