LU-2501 osc: hold cli list lock for osc_exit_cache

Somehow it's not held before which hit the assert.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I48e50c3e8df197ed5d17ceecf41e995cc7d87f69
Reviewed-on: http://review.whamcloud.com/4834
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Oleg Drokin <green@whamcloud.com>