There are some corner cases that can create an unmapped, wired region in
a user address space:

• msync(MS_INVALIDATE)
• mprotect(PROT_NONE) -> mprotect(PROT_READ)
• truncation of a vnode/shm object to a smaller size and back to a larger size

A fault on such a region can trigger preemptive superpage creation
with pmap_enter(PMAP_ENTER_WIRED, psind=1). However, the various
pmaps do not handle this properly and will create a wired superpage
without a leaf page in the pmap's radix tree, so demotion may fail
and cause the address range to be unmapped.

I am not yet sure that we should fix all of the pmaps to handle this,
but for now just make it impossible.