aio: prevent double free in ioctx_alloc

ioctx_alloc() calls aio_setup_ring() to allocate a ring. If aio_setup_ring()
fails to do so it would call aio_free_ring() before returning, but
ioctx_alloc() would call aio_free_ring() again causing a double free of
the ring.

This is easily reproducible from userspace.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -610,7 +610,6 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
 err_cleanup:
 	aio_nr_sub(ctx->max_reqs);
 err:
-	aio_free_ring(ctx);
 	free_percpu(ctx->cpu);
 	free_percpu(ctx->reqs.pcpu_count);
 	free_percpu(ctx->users.pcpu_count);