ION: Sys_heap: Add cached pool to spead up cached buffer alloc

Add ion cached pool in system heap. This patch add a cached pool
in system heap. It has a great improvement of alloc for cached
buffer.

With memory pressue alloc test 800MB in userspace used iontest.
The result avg is 577ms. Without patch it's avg is about 883ms.

v1: Makes the cached buffer zeroed before going to pool
v2: Add cached param in pool to distinguish wheather need to flush
    cache at a fresh alloc.
    Rework the shrink function.

Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Xia Qing <saberlily.xia@hisilicon.com>
Reviewed-by: Fu Jun <oliver.fu@hisilicon.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>