Assemble.c: respect force flag.

Assemble.c: respect force flag.

If the array is dirty handler will set resync_start to 0 to inform kernel
that resync is needed. RWH affects only raid456 module, for other
levels array will be started even array is degraded and resync cannot be
performed.

Force is really meaningful for raid456. If array is degraded and resync
is requested, kernel will reject an attempt to start the array. To
respect force, it has to be marked as clean (this will be done for each
array without PPL) and remove the resync request (only for raid 456).
Data corruption may occur so proper warning is added.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>