sctp: improve handling of address changes

Identify interfaces consistenly by the pair of the ifn pointer
and the index.
This avoids a use after free when the ifn and or index was reused.

Reported by: bz, pho, and others

(cherry picked from commit 523913c94371ab50a8129cbab820394d25f7a269)
(cherry picked from commit 331db93815afb49b01f269aeff0fe899acd47455)

Approved by: re (cperviva)