Few more fixes to the new pagetable page allocation code.

[C. Jayachandran]

The attached changes
(http://people.freebsd.org/~jchandra/for-review/pmap-free-pte-page.diff)
finally seems to get rid of the occasional hangs I saw during testing.

The first part fixes a lock order reversal (in
pmap_remove_pages...pmap_release_pte_page...uma_zfree_arg...kmem_malloc).
 The pmap and pagequeue locks have to be released before calling
uma_zfree().  The second part takes the the pagequeue lock before
changing the page fields of the pte page, I'm not sure this is
necessary.

Please let me know you comments. With these changes, I can get 'make
-j128 buildworld' consistently work on a 32 cpu setup.

Thanks,
JC.
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