xfs: remove unused calculation in xfs_dir2_sf_addname()

The "add_entsize" calculated here is never used.
"incr_isize" accounts for the inode expansion of the
old entries + parent + new entry all by itself.

Once we've removed add_entsize there, it's just a pointless
intermediate variable elsewhere, so remove it.
For that matter, old_isize is gratuitous too, so nuke that.

And add a few comments so the magic "+1's" and "+2's" make
a bit more sense.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>