tcp: Make use of enum for sack_changed
No functional change.
Reviewed By: tuexen, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43346
(cherry picked from commit 893ed42eca32f7b5e3a5b8cef3f364be0d9cf3ef)