RDMA/addr: Fix build breakage when IPv6 is disabled

Commit 38617c64

 ("RDMA/addr: Add support for translating IPv6
addresses") broke the build when CONFIG_IPV6=n, because the ib_addr
module unconditionally attempted to call ipv6_chk_addr() and other
IPv6 functions that are not defined when IPv6 is disabled.  Fix this
by only building IPv6 support if CONFIG_IPV6 is turned on, and
add a Kconfig dependency to prevent the ib_addr code from being built
in when IPv6 is built modular.

Signed-off-by: Roland Dreier <rolandd@cisco.com>