md/raid5: more incorrect BUG_ON in handle_stripe_fill.

It is not incorrect to call handle_stripe_fill() when
a batch of full-stripe writes is active.
It is, however, a BUG if fetch_block() then decides
it needs to actually fetch anything.

So move the 'BUG_ON' to where it belongs.

Signed-off-by: NeilBrown <neilb@suse.de>
Fixes: 59fc630b ("RAID5: batch adjacent full stripe write")