IB/core: Rename event_handler_lock to qp_open_list_lock

This lock is used to protect the qp->open_list linked list. As a side
effect it seems to also globally serialize the qp event_handler, but it
isn't clear if that is a deliberate design.

Link: https://lore.kernel.org/r/20191212113024.336702-5-leon@kernel.org


Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>