LU-2948 osc: keep trying to shrink more LRU slots

LU-2948 osc: keep trying to shrink more LRU slots

It used to try once. It can enter into a livelock state as we can
see it in this ticket.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: If603686d1d9fa7cf6513143fcc6ef962cfea9863
Reviewed-on: http://review.whamcloud.com/5760
Tested-by: Hudson
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>