TCP: send full initial window when timestamps are in use

The fastpath in tcp_output tries to send out
full segments, and avoid sending partial segments by
comparing against the static t_maxseg variable.
That value does not consider tcp options like timestamps,
while the initial window calculation is using
the correct dynamic tcp_maxseg() function.

Due to this interaction, the last, full size segment
is considered too short and not sent out immediately.

Reviewed by:	tuexen
MFC after:	2 weeks
Sponsored by:	NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D26478

newlib/libc/sys/rtems/include/netinet/tcp.h

@@ -80,6 +80,8 @@ struct tcphdr {
 	u_short	th_urp;			/* urgent pointer */
 };
 
+#define	PADTCPOLEN(len)		((((len) / 4) + !!((len) % 4)) * 4)
+
 #define	TCPOPT_EOL		0
 #define	   TCPOLEN_EOL			1
 #define	TCPOPT_PAD		0		/* padding after EOL */