Permit vm_page_wire() to be called on pages not belonging to an object.

For such pages ref_count is effectively a consumer-managed field, but
there is no harm in calling vm_page_wire() on them.
vm_page_unwire_noq() handles them as well. Relax the vm_page_wire()
assertions to permit this case which is triggered by some out-of-tree
code. [1]

Also guard a conditional assertion with INVARIANTS. Otherwise the
conditions are evaluated even though the result is unused. [2]

Reported by: bz, cem [1], kib [2]
Reviewed by: dougm, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26173