vrf: Do not attempt to create IPv6 mcast rule if IPv6 is disabled

A user reported that vrf create fails when IPv6 is disabled at boot using
'ipv6.disable=1':
   https://bugzilla.kernel.org/show_bug.cgi?id=204903



The failure is adding fib rules at create time. Add RTNL_FAMILY_IP6MR to
the check in vrf_fib_rule if ipv6_mod_enabled is disabled.

Fixes: e4a38c0c ("ipv6: add vrf table handling code for ipv6 mcast")
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Patrick Ruddy <pruddy@vyatta.att-mail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>