tcp: improve handling of SYN segments in SYN-SENT state

Ensure that the stack does not generate a DSACK block for user
data received on a SYN segment in SYN-SENT state.

Reviewed by: rscheff
Differential Revision: https://reviews.freebsd.org/D29376
Sponsored by: Netflix, Inc.

(cherry picked from commit 40f41ece765dc0b0907ca90796a1af4f4f89b2a0)