Linux 3.0 compat, shrinker compatibility

To accomindate the updated Linux 3.0 shrinker API the spl
shrinker compatibility code was updated. Unfortunately, this
couldn't be done cleanly without slightly adjusting the comapt
API. See spl commit a55bcaad181096d764e12d847e3091cd7b15509a.

This commit updates the ZFS code to use the slightly modified
API. You must use the latest SPL if your building ZFS.