LU-1428 ldlm: fix a race in ldlm_lock_destroy_internal

LU-1428 ldlm: fix a race in ldlm_lock_destroy_internal

ldlm_lock::l_exp_hash should be protected by internal lock of
cfs_hash, but we called cfs_hlist_unhashed(lock::l_exp_hash)
w/o holding cfs_hash lock in ldlm_lock_destroy_internal,
which means if someone called ldlm_lock_cancel on a lock while
export::exp_lock_hash is in progress of rehashing (thread context of
cfs_workitem), there could be tiny window between deleting this lock
from bucket[A] and re-adding it to bucket[B] of l_exp_hash, and
cfs_hlist_unhashed(lock::l_exp_hash) will return 1 in this window,
then we destroyed a lock but left it on l_exp_hash forever because
lock::l_destroyed has been set to 1 and ldlm_lock_destroy_internal()
wouldn't be able to remove the lock from l_exp_hash even it's called
infinite times in ldlm_cancel_locks_for_export_cb().

This patch also added some debug information to
ldlm_cancel_locks_for_export_cb in case this patch can't fix this
problem.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: Ia0932658b3f085a55535e36bee4fb833e74fa242
Reviewed-on: http://review.whamcloud.com/3028
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>