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")

(cherry picked from commit 99c58ad021b2f7dc0496e16d313c5e28a552f0d0)