LU-2398 osc: ignore layout for osc_lru_shrink()

osc_lru_shrink() doesn't need to check verify layout change
because layout can't be destroyed if a page already exists.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I034c15cbecfaf1491d7d7fcb992b74e3d94d9fa5
Reviewed-on: http://review.whamcloud.com/4701
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>