This patch picked up the code that was used to calculate the number of
the items for which we need reserve space, and we will use it in the next
patch.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>