Add some isync()s related to the 64-bit MMU scratch page to avoid race
conditions on its invalidation.

Sync: merge r216133 through r216338 from ^/head.

svn+ssh://svn.freebsd.org/base/head@216199

Add an abstraction layer to the 64-bit AIM MMU's page table manipulation
logic to support modifying the page table through a hypervisor. This
uses KOBJ inheritance to provide subclasses of the base 6

Add an abstraction layer to the 64-bit AIM MMU's page table manipulation
logic to support modifying the page table through a hypervisor. This
uses KOBJ inheritance to provide subclasses of the base 64-bit AIM MMU
class with additional methods for page table manipulation.

Many thanks to Peter Grehan for suggesting this design and implementing
the MMU KOBJ inheritance mechanism.

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