LU-11089 obd: rename lu_keys_guard to lu_context_remembered_guard

LU-11089 obd: rename lu_keys_guard to lu_context_remembered_guard

The only remaining use of lu_keys_guard is to protect the
lu_context_remembers linked list, and always write_lock()
is used.
So rename it to reflect this, and change to a spinlock.
We move keys_fini() out of the locked region in
lu_context_fini() - once we have removed the context from
the lc_remembers list, there can no longer be a race.

Linux-commit: dc8aaaca0062878c2fbad9df1b9ac3e85cad8630

Change-Id: Id66930e073d5351a96b139f2fc1a8007841de728
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/33673
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>