Fix kernel module refcount handling

md loads raidX modules and increments module refcount each time level
has changed but does not decrement it. You are unable to unload raid0
module after reshape because raid0 reshape changes level to raid4
and back to raid0.

Signed-off-by: Aleksey Obitotskiy <aleksey.obitotskiy@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>