Don't hold SLOB lock when freeing the page. Reduces lock hold width. See
the following thread for discussion of the bug:

  http://marc.info/?l=linux-kernel&m=123709983214143&w=2Reported-by: NIngo Molnar <mingo@elte.hu>
Acked-by: NMatt Mackall <mpm@selenic.com>
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>