/* make amap larger */
/*
* amap_extend: extend the size of an amap (if needed)
*
* => called from uvm_map when we want to extend an amap to cover
* a new mapping (rather than allocate a new one)
* => amap should be unlocked (we will lock it)
* => to safely extend an amap it should have a reference count of
* one (thus it can't be shared)
*/