md/raid5: Move code for finishing a reconstruction into handle_stripe.

md/raid5: Move code for finishing a reconstruction into handle_stripe.

Prior to commit 5bd75a0625f6a the code in handle_stripe5 and
handle_stripe6 to "Finish reconstruct operations initiated by the
expansion process" was identical.
That commit added an identical stanza of code to each function, but in
different places.  That was careless.

The raid5 code was correct, so move that out into handle_stripe and
remove raid6 version.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Namhyung Kim <namhyung@gmail.com>