kernel -- vm locking: Add vm_page_(un)lock and vm_object_(un)lock.

kernel -- vm locking: Add vm_page_(un)lock and vm_object_(un)lock.

Each vm_object and vm_page are associated with a token; for vm_pages,
we use a pool token; for objects, a per-object token. For vm_pages,
the token will interlock access to the pv_chain, at least.

Also remove per-vm_object range locks. They were unused.