IB/cm: Introduce and use helper function to get cm_port from path

Introduce and use helper function get_cm_port_from_path() to get
cm_port based on the the path record entry.

Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>