MFC r298769, r299315

r298769

tcp/syncache: Set flowid and hash type properly for SYN|ACK

So the underlying drivers can use it to select the sending queue
properly for SYN|ACK instead of rolling their own hash.

Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6120

r299315

tcp/syncache: Add comment for syncache_respond

Suggested by: hiren, hps
Reviewed by: sbruno
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6148