netfilter.c: use literal string for printf format

TL;DR: this allows to check if printf argument types are valid.

Apparently, gcc is not able to check if the printf arguments
are in sync with the format string, it a string is not a literal.
This can be seen by compiling the code with -Wformat-nonliteral:

  CC       criu/netfilter.o
criu/netfilter.c: In function ‘nf_connection_switch_raw’:
criu/netfilter.c:80:4: error: format not a string literal, argument
types not checked [-Werror=format-nonliteral]
    dip, (int)dst_port, sip, (int)src_port);

Unfortunately we can't just add -Wformat-nonliteral to CFLAGS as there
is at least one other place in the code what uses non-literal string
as a format string for printf-like function. In this very case, though,
there is no need to use a non-literal, so change it to a define.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/netfilter.c

@@ -21,8 +21,8 @@ static char buf[512];
  * ANy brave soul to write it using xtables-devel?
  */
 
-static const char *nf_conn_cmd = "%s %s -t filter %s %s --protocol tcp "
-	"--source %s --sport %d --destination %s --dport %d -j DROP";
+#define NF_CONN_CMD	"%s %s -t filter %s %s --protocol tcp " \
+	"--source %s --sport %d --destination %s --dport %d -j DROP"
 
 static char iptable_cmd_ipv4[] = "iptables";
 static char iptable_cmd_ipv6[] = "ip6tables";
@@ -73,7 +73,7 @@ static int nf_connection_switch_raw(int family, u32 *src_addr, u16 src_port,
 		return -1;
 	}
 
-	snprintf(buf, sizeof(buf), nf_conn_cmd, cmd,
+	snprintf(buf, sizeof(buf), NF_CONN_CMD, cmd,
 			kdat.has_xtlocks ? "-w" : "",
 			lock ? "-A" : "-D",
 			input ? "INPUT" : "OUTPUT",