Make page busy state deterministic on free. Pages must be xbusy when

removed from objects including calls to free.  Pages must not be xbusy
when freed and not on an object.  Strengthen assertions to match these
expectations.  In practice very little code had to change busy handling
to meet these rules but we can now make stronger guarantees to busy
holders and avoid conditionally dropping busy in free.

Refine vm_page_remove() and vm_page_replace() semantics now that we have
stronger guarantees about busy state.  This removes redundant and
potentially problematic code that has proliferated.

Discussed with:	markj
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D22822

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