RDMA/cma: Fix unknown symbol when CONFIG_IPV6 is not enabled

If IPV6 has not been enabled in the underlying kernel, we must avoid
calling IPV6 procedures in rdma_cm.ko.

This requires using "IS_ENABLED(CONFIG_IPV6)" in "if" statements
surrounding any code which calls external IPV6 procedures.

In the instance fixed here, procedure cma_bind_addr() called
ipv6_addr_type() -- which resulted in calling external procedure
__ipv6_addr_type().

Fixes: 6c26a771 ("RDMA/cma: fix IPv6 address resolution")
Cc: <stable@vger.kernel.org> # v4.2+
Cc: Spencer Baugh <sbaugh@catern.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>