tcp: LRO code to deal with all 12 TCP header flags

TCP per RFC793 has 4 reserved flag bits for future use. One
of those bits may be used for Accurate ECN.
This patch is to include these bits in the LRO code to ease
the extensibility if/when these bits are used.

Reviewed By: hselasky, rrs, #transport
Sponsored by:        NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34127

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

@@ -55,12 +55,12 @@ struct tcphdr {
 	tcp_seq	th_seq;			/* sequence number */
 	tcp_seq	th_ack;			/* acknowledgement number */
 #if BYTE_ORDER == LITTLE_ENDIAN
-	u_char	th_x2:4,		/* (unused) */
+	u_char	th_x2:4,		/* upper 4 (reserved) flags */
 		th_off:4;		/* data offset */
 #endif
 #if BYTE_ORDER == BIG_ENDIAN
 	u_char	th_off:4,		/* data offset */
-		th_x2:4;		/* (unused) */
+		th_x2:4;		/* upper 4 (reserved) flags */
 #endif
 	u_char	th_flags;
 #define	TH_FIN	0x01