LU-11287 ldlm: update l_blocking_lock under lock

LU-11287 ldlm: update l_blocking_lock under lock

Update l_blocking_lock under with locking to prevent race
between lock_handle_convert0() and ldlm_work_bl_ast() code.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I881a1daf6f3b09677abcd6a85f6891d409926cc8
Reviewed-on: https://review.whamcloud.com/33124
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>