RDMA/ocrdma: Fix build with IPV6=n

When IPV6 is not enabled:

    ERROR: "register_inet6addr_notifier" [drivers/infiniband/hw/ocrdma/ocrdma.ko] undefined!
    ERROR: "unregister_inet6addr_notifier" [drivers/infiniband/hw/ocrdma/ocrdma.ko] undefined!

Fix this by wrapping the inet6 calls in #ifdef IPV6.  Also make the
ocrdma module depend on (IPV6 || IPV6=n) to forbid the case of modular
ipv6 but built-in ocrdma (which can't work, because ocrdma calls ipv6
functions).

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Roland Dreier <roland@purestorage.com>