LU-4456 osp: extra check for opd_pre

LU-4456 osp: extra check for opd_pre

1. Add extra check for opd_pre in statfs_interpret, in case
opd_pre has been freed before the callback.

2. switch the sync_fini and pre_fini, so opd_pre will be freed
after all of the possible access has been stopped.

3. opd_pre_waitq will be accessed in several update threads,
osp_precreate, osp_statfs_timer_cb, statfs_interrupt, move
it to osp_device to make sure it is accessiable even after
osp_pre is freed.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I5c73cb52e2406ed03570fc3471111c409e6fe08f
Reviewed-on: http://review.whamcloud.com/8890
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>