IB/core: Fix potential memory leak while creating MAD agents

commit 6e88e672b69f0e627acdae74a527b730ea224b6b upstream.

If the MAD agents isn't allowed to manage the subnet, or fails to register
for the LSM notifier, the security context is leaked. Free the context in
these cases.

Fixes: 47a2b338 ("IB/core: Enforce security on management datagrams")
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reported-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>