IB/uverbs: Allow CQ moderation with modify CQ

Uverbs support in modify_cq for CQ moderation only.
Gives ability to change cq_max_count and cq_period.
CQ moderation enhance performance by moderating the number
of CQEs needed to create an event instead of application
having to suffer from event per-CQE.
To achieve CQ moderation the application needs to set cq_max_count
and cq_period.
cq_max_count - defines the number of CQEs needed to create an event.
cq_period - defines the timeout (micro seconds) between last
            event and a new one that will occur even if
	    cq_max_count was not satisfied

Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>