RDMA/cma: Increase the IB CM retry count in CMA

3 seems like a low number of IB Communication Manager retries to set;
we see connections failing under stress, and in any case 3 just looks
like an arbitrary number.  15 is the max value allowed by the
InfiniBand spec.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>