Responding properly to SYN-ACK

[rrs]

Summary

When in the SYN-SENT state bbr and rack will not properly send an ACK but instead start the D-ACK timer. This
causes so_reuseport_lb_test to fail since it slows down how quickly the program runs until the timeout occurs
and fails the test.

Test Plan

Apply the fix and make sure that when bbr or rack is set as the default stack
so_reuseport_lb_test passes.