Add missing check for failure of cm_init_av_by_path

Fixes: e1444b5a ("IB/cm: Fix automatic path migration support")
Reported-by: Slava Shwartsman <slavash@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>