tcp: fix a RACK socket buffer lock issue

Fix a missing socket buffer unlocking of the socket receive buffer.

Reviewed by: gallatin, rrs
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D30402

(cherry picked from commit 9bbd1a8fcb13928cd4b6cfddf0a8359d5dc97451)