Add an assertion to pmap_enter().

When modifying an existing managed mapping, we should find a PV entry
for the old mapping. Verify this.

Before r335784 this would have been implicitly tested by the fact that
we always freed the PV entry for the old mapping.

Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17626