Apply some band-aid to pmap_activate() for PR kern/41058:

There's a corner case here which can leave turds in the cache as
reported in kern/41058. They're probably left over during tear-down and
switching away from an exiting process. Until the root cause is identified
and fixed, zap the cache when switching pmaps. This will result in a few
unnecessary cache flushes, but that's better than silently corrupting data.

Also remove an extraneous return statement in pmap_page_protect() which
crept in during the matt-armv6 merge.

(scw)