IB/core: Unregister notifier before freeing MAD security

commit d60667fc398ed34b3c7456b020481c55c760e503 upstream.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>