Add kmem cache defragmentation API

[behlendorf]

There are now consumers of the kmem cache defragmentation API, as such we should fully implement it. Basically, this will involve implementing the kmem_cache_set_move() API function to register the move callback for the slab. Then there going to need to be a few new data structure to track this. Finally, we will need to safely add all the hooks to the existing slab code to ensure it is called appropriately and the returns code are honors. Now this is going to be a little trickier than it sounds since the slab implementation is fair complicated any racy, but it does need to be done at some point. We absolutely will need to add several regression tests to make sure we got it right.

See commit 2b35430.

[behlendorf]

This may be slightly more important now with the addition of emergency objects since this would provide a clean mechanism for migrating those objects to existing slabs.

[behlendorf]

Closing. Implementing this hasn't proven to be needed.