From: Nick Piggin <npiggin@suse.de>

Allocate a compound page for the user mapping instead of tweaking the page
refcounts.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NDave Airlie <airlied@linux.ie>