IB/core: Fix use after free of ifa

When using ifup/ifdown while executing enum_netdev_ipv4_ips,
ifa could become invalid and cause use after free error.
Fixing it by protecting with RCU lock.

Fixes: 03db3a2d ('IB/core: Add RoCE GID table management')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>