tcp: fix format of sysctl variable

The format for CTLTYPE_UINT is "IU" instead of "UI" as specified
in sysctl.9.

Reviewed by:		cc, zlei
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D46408

sys/netinet/tcp_syncache.c

@@ -207,7 +207,7 @@ sysctl_net_inet_tcp_syncache_rexmtlimit_check(SYSCTL_HANDLER_ARGS)
 SYSCTL_PROC(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit,
     CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
-    sysctl_net_inet_tcp_syncache_rexmtlimit_check, "UI",
+    sysctl_net_inet_tcp_syncache_rexmtlimit_check, "IU",
     "Limit on SYN/ACK retransmissions");
 
 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;