IB/core: Use hop-limit from IP stack for RoCE

Previously, IPV6_DEFAULT_HOPLIMIT was used as the hop limit value for
RoCE. Fixing that by taking ip4_dst_hoplimit and ip6_dst_hoplimit as
hop limit values.

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