net: sctp: refactor active path selection

This patch just refactors and moves the code for the active
path selection into its own helper function outside of
sctp_assoc_control_transport() which is already big enough.
No functional changes here.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>