LU-8957 obdclass: fix exp_nid_hash race

LU-8957 obdclass: fix exp_nid_hash race

Move exp_nid_hash unhash operation under exp_lock. To avoid
parallel export disconnect may race in there.

Change-Id: I02a9cf550b4edee7c42ba90d3f3926cd2584c24b
Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-on: https://review.whamcloud.com/24434
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>