MFC r204268:

MFC r204268:

Close a race involving the OEA64 scratchpage. When the scratch page's
physical address is changed, there is a brief window during which its PTE
is invalid. Since moea64_set_scratchpage_pa() does not and cannot hold
the page table lock, it was possible for another CPU to insert a new PTE
into the scratch page's PTEG slot during this interval, corrupting both
mappings.

Solve this by creating a new flag, LPTE_LOCKED, such that
moea64_pte_insert will avoid claiming locked PTEG slots even if they
are invalid. This change also incorporates some additional paranoia
added to solve things I thought might be this bug.

Reported by:	linimon

git-svn-id: svn://svn.freebsd.org/base/stable/8@205379 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f