A required unlock_page will be missed in a very rare (but possible) race
condition. Acked by Hugh, who says:

	It'll be hard to hit because of the additional page_mapped test above,
	with truncate unmapping ptes from mms before it advances to removing
	pages from cache; but nothing to prevent it happening.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>