LU-8918 obdclass: hoist locking in lu_context_exit()

LU-8918 obdclass: hoist locking in lu_context_exit()

Hoist lu_keys_guard locking out of the for loop in lu_context_exit().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I2f79a1321a48d8a459b4f83a7996ca2ae9a8e4bb
Reviewed-on: https://review.whamcloud.com/24217
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>