LU-12719 obdclass: serialize lwp list access

LU-12719 obdclass: serialize lwp list access

lustre_sb_info.lsi_lwp_list should be acessed with lock, and
some place may sleep, change lsi_lwp_lock from spinlock to mutex.

Lustre-change: https://review.whamcloud.com/36003
Lustre-commit: 875252d59924ad09db8de9f0fbb611788a0b9c78

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ifc3622eb28cd6cf49661b14fc10e98aa689a58dc
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36349
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>