IB/core: Separate CQ handle in SRQ context

Before this change CQ attached to SRQ was part of XRC specific extension.
Moving CQ handle out makes it available to other types extending SRQ
functionality.

Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Reviewed-by: Yossi Itigin <yosefe@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>