[tcp] Record ts_recent on first received packet

[tcp] Record ts_recent on first received packet

Commit 6861304 ("[tcp] Handle out-of-order received packets")
introduced a regression in which ts_recent would not be updated until
the first packet is received in the ESTABLISHED state, i.e. the
timestamp from the SYN+ACK packet would be ignored.  This causes the
connection to be dropped by strictly-conforming TCP peers, such as
FreeBSD.

Fix by delaying the timestamp window check until after processing the
received SYN flag.

Reported-by: winders@sonnet.com
Signed-off-by: Michael Brown <mcb30@ipxe.org>