MRP-2085 mgc: rmmod mgc doesn't cause panic

mgc could be unloaded inspite of ll_cfg_requeue thread
is not finished. It is possible when mgc unloading starts
before the 1st execution of ll_cfg_requeue, i.e. thread
was created but sheduler has not started it.

Change-Id: I8644ab83e920813c115488c6a64831f5252e29e8
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@seagate.com>
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/4067
Tested-by: Jenkins
Reviewed-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Reviewed-by: Alexander Boyko <alexander_boyko@xyratex.com>
Tested-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>