IB/core: Extend ib_uverbs_create_cq

ib_uverbs_ex_create_cq follows the extension verbs
mechanism. New features (for example, CQ creation flags
field which is added in a downstream patch) could used
via user-space libraries without breaking the ABI.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>