percpu: fix locking regression in the failure path of pcpu_alloc()

While updating locking, b38d08f3 ("percpu: restructure locking")
broke pcpu_create_chunk() creation path in pcpu_alloc().  It returns
without releasing pcpu_alloc_mutex.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>