LU-499 grant/cancel_rate are static when OST is idle

LU-499 grant/cancel_rate are static when OST is idle

ldlm_pool_recalc() shouldn't be skipped if namespace->ns_bref eqauls
zero, instead a flag ns_stopping is added to mark ldlm namespace is
being freed.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: Ic6485c34ec3e9868ae531a4dc25aee969c374eb5
Reviewed-on: http://review.whamcloud.com/1185
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>