LU-11775 obdclass: protect imp_sec using rwlock_t

LU-11775 obdclass: protect imp_sec using rwlock_t

We've seen spinlock contention on imp_lock in
sptlrpc_import_sec_ref(), introduce a new rwlock
imp_sec_lock to protect imp_sec instead of using imp_lock.

This patch also removes imp_sec_mutex from obd_import,
which is not needed, to avoid confusion between
imp_sec_lock/mutex.

Test-Parameters: testlist=sanity-sec envdefinitions=SHARED_KEY=true
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I3be3a6d1225888134bf7dd58e4b05b864f8415b4
Reviewed-on: https://review.whamcloud.com/33861
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>