IB/core: Init subsys if compiled to vmlinuz-core

Once infiniband is compiled as a core component its subsystem must be
enabled before device initialization. Otherwise there is a NULL pointer
dereference during mlx4_core init, calltrace:
->device_add
  if (dev->class) {
     deref  dev->class->p =>NULLPTR

#Config
CONFIG_NET_DEVLINK=y
CONFIG_MAY_USE_DEVLINK=y
CONFIG_MLX4_EN=y
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/device.c

@@ -1253,5 +1253,5 @@ static void __exit ib_core_cleanup(void)
 
 MODULE_ALIAS_RDMA_NETLINK(RDMA_NL_LS, 4);
 
-module_init(ib_core_init);
+subsys_initcall(ib_core_init);
 module_exit(ib_core_cleanup);