Invalidate the mapping before updating its physical address.

Doing so ensures that all threads sharing the pmap have a consistent
view of the mapping. This fixes the problem described in the commit
log messages for r329254 without the overhead of an extra fault in the
common case. Once other pmap_enter() implementations are similarly
modified, the workaround added in r329254 can be removed, reducing the
overhead of CoW faults.

With this change we can reuse the PV entry from the old mapping,
potentially avoiding a call to reclaim_pv_chunk(). Otherwise, there is
nothing preventing the old PV entry from being reclaimed. In rare
cases this could result in the PTE's page table page being freed,
leading to a use-after-free of the page when the updated PTE is written
following the allocation of the PV entry for the new mapping.

Reported and tested by: pho
Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D16005