IB/core: Unregister notifier before freeing MAD security

mainline inclusion
from mainline-5.0
commit d60667fc
category: bugfix
bugzilla: 12231
CVE: NA

-------------------------------------------------

If the notifier runs after the security context is freed an access of
freed memory can occur.

Fixes: 47a2b338 ("IB/core: Enforce security on management datagrams")
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Reviewed-by: Wenan Mao <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>