tcp: fix t_flags2 collision

Fix the collision of TF2_IPSEC_TSO and TF2_NO_ISS_CHECK

Fixes: 646c28ea80cb ("tcp: improve SEG.ACK validation")
Sponsored by: Netflix, Inc.

(cherry picked from commit 52eacec95d54205962a3d6902e18e41fce8b4e79)