Revert to zero'ing the objects we pull out of our objcaches ourselves.

Revert to zero'ing the objects we pull out of our objcaches ourselves.
Having it done by objcache_get() incurs too much of a penalty on the hot
path of it, that it isn't warranted, especially considering the fact that
pulling zero'ed objects out of it isn't taking advantage of the objcache
model. However, there are arguments for using it, like growability in
interrupt context (zones can't).