sctp: compute address flags only for IPv6 addresses

Only call sctp_gather_internal_ifa_flags() for IPv6 addresses and
also compile this code only, when IPv6 is supported.
This fixes the compilation of IPv4 only kernels.

Reported by: bz@
Fixes: 6ab4b0c0df57 ("sctp: initilize local address flags correctly")
MFC after: 3 days