In the rare case that vm_page_cache() actually frees the given page, it must first ensure that the page is no longer mapped.  This is trivially accomplished by calling pmap_remove_all() a little earlier in vm_page_cache().  While I'm in the neighborbood, make a related panic message a little more useful.
In the rare case that vm_page_cache() actually frees the given page,
it must first ensure that the page is no longer mapped.  This is
trivially accomplished by calling pmap_remove_all() a little earlier
in vm_page_cache().  While I'm in the neighborbood, make a related
panic message a little more useful.

Approved by:	re (kensmith)
Reported by:	Peter Holm and Konstantin Belousov
Reviewed by:	Konstantin Belousov

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