Assemble: fix busy detection

Assemble: fix busy detection

Use mddev_busy() as GET_ARRAY_INFO can succeed on 'clear' arrays.

Ran into this after an encountering a case where mdadm -Ss ended in
segfault (missing check for NULL return from map_by_devnum() in
sles11:Manage.c).  So, tried to stop the array by hand with echo clear >
md/array_state, after which I could not reassemble since GET_ARRAY_INFO
was succeeding.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>