Make ib allocation size function of cs size

[j.glisse at gmail.com]

Two following patch are on top of
http://cgit.freedesktop.org/~glisse/linux

They make the ib allocation size a function of the cs size, this
allow to avoid wasting pool space and avoid to trigger fence_wait
in ib_get. I am still evaluating how much fence_wait we avoid
with this.

Cheers,
Jerome