Btrfs: optimize leaf_space_used

This gets called at least 4 times for every level while adding an object,
and it involves 3 kmapping calls, which on my box take about 5us a piece.
So instead use a token, which brings us down to 1 kmap call and makes this
function take 1/3 of the time per call.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>