LU-6800 obdclass: change spinlock of key to rwlock

LU-6800 obdclass: change spinlock of key to rwlock

Most of the time, keys are never changed. So rwlock might be
better for the concurrency of key read.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I5c86c7333884a44ec8e9017d532d6eea5a119cfa
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Reviewed-on: http://review.whamcloud.com/15558
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>