MFC: r269051

Copying pages via temporary mappings in the !DMAP case of pmap_copy_pages()
involves updating the corresponding page tables followed by accesses to the
pages in question. This sequence is subject to the situation exactly described
in the "AMD64 Architecture Programmer's Manual Volume 2: System Programming"
rev. 3.23, "7.3.1 Special Coherency Considerations" [1, p. 171 f.]. Therefore,
issuing the INVLPG right after modifying the PTE bits is crucial (see also
r269050, MFCed to stable/10 in r269235).
For the amd64 PMAP code, the order of instructions was already correct. The
above fact still is worth documenting, though.

1: http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2012/10/24593_APM_v21.pdf

Reviewed by:	alc
Sponsored by:	Bally Wulff Games & Entertainment GmbH

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