md/raid5: add missing spin_lock_init.

commit b17459c0
   raid5: add a per-stripe lock

added a spin_lock to the 'stripe_head' struct.
Unfortunately there are two places where this struct is allocated
but the spin lock was only initialised in one of them.

So add the missing spin_lock_init.

Signed-off-by: NeilBrown <neilb@suse.de>