LU-11301 target: add lock in sub_trans_stop_cb()

LU-11301 target: add lock in sub_trans_stop_cb()

sub_trans st_committed and st_stopped flags take different bit of
the same memory address, so both of them should be set with lock,
but sub_trans_stop_cb() doesn't, which may cause it overriden.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ic3d7d7281b3cf9bd20702be944e14f35200318f1
Reviewed-on: https://review.whamcloud.com/33169
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>