md/raid5: new alloc_stripe() to allocate an initialize a stripe.

The new batch_lock and batch_list fields are being initialized in
grow_one_stripe() but not in resize_stripes().  This causes a crash
on resize.

So separate the core initialization into a new function and call it
from both allocation sites.

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