lxc_user_nic: fix get_mtu() error handling

get_mtu() returns int, but "mtu" variable has unsigned int type.
It leads to logical error in error handling, which can end up
with strange -EINVAL error in lxc_veth_create(), cause (mtu > 0)
condition is met, but negative "mtu" value is too large when set
as mtu for network device.

Issue #4232

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

src/lxc/cmd/lxc_user_nic.c

@@ -502,7 +502,7 @@ static int get_mtu(char *name)
 
 static int create_nic(char *nic, char *br, int pid, char **cnic)
 {
-	unsigned int mtu = 1500;
+	int mtu = 1500;
 	int ret;
 	char veth1buf[IFNAMSIZ], veth2buf[IFNAMSIZ];
 
@@ -520,11 +520,11 @@ static int create_nic(char *nic, char *br, int pid, char **cnic)
 
 	if (strcmp(br, "none"))
 		mtu = get_mtu(br);
-	if (!mtu)
+	if (mtu <= 0)
 		mtu = 1500;
 
 	/* create the nics */
-	ret = instantiate_veth(veth1buf, veth2buf, pid, mtu);
+	ret = instantiate_veth(veth1buf, veth2buf, pid, (unsigned int)mtu);
 	if (ret < 0) {
 		usernic_error("%s", "Error creating veth tunnel\n");
 		return -1;