sctp: #ifdef INET-only and INET6-only variables.

Duplicating the SCTP_PCB_FLAGS_BOUND_V6 check made the #ifdef's
simpler than applying #ifdef's directly to the original code. Modern
compilers should cache the result rather than testing the flag twice.

(cherry picked from commit 29a843177e108c688d7daaccac6ada4d16f01dca)