powerpc64/mmu: Use a SLIST for the PVO delete list, instead of a RB_TREE

[jhibbits]

Summary

Although it's convenient to reuse the pvo_plist for deletion, RB_TREE
insertion and removal is not free, and can result in a lot of extra work
to rebalance the tree. Instead, use a SLIST as a LIFO delete queue,
which gives us almost free insertion, deletion, and traversal.

[bdragon]

Boots on ELFv1 and ELFv2. Doing a buildworld to excercise it a bit.

[bdragon]

ELFv1 and ELFv2 buildworld worked fine.

[luporl]

LGTM