Avoid unnecessary atomic operations for clearing PGA_WRITEABLE in pmap_remove_pages().  This reduces pmap_remove_pages()'s running time by 4 to 11% in my tests.
Avoid unnecessary atomic operations for clearing PGA_WRITEABLE in
pmap_remove_pages().  This reduces pmap_remove_pages()'s running time by
4 to 11% in my tests.

MFC after:	1 week

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