tools/hotplug: validate mtu read from xenstore against RFC 791 minimum

RFC 791 sets the minimum acceptable mtu to 68:

    Every internet module must be able to forward a datagram of 68
    octets without further fragmentation.  This is because an internet
    header may be up to 60 octets, and the minimum fragment is 8 octets.

In the Linux kernel source:

include/net/ip.h:#define IPV4_MIN_MTU           68              /* RFC 791 */

While there is no reason to expect a value <68 to be read from xenstore or
from the bridge make the value test require a number >= this minimum.

Signed-off-by: James Dingwall <james@dingwall.me.uk>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

tools/hotplug/Linux/xen-network-common.sh

@@ -167,7 +167,7 @@ set_mtu () {
             log debug "$bridge MTU is $mtu"
         fi
     fi
-    if [ -n "$mtu" ] && [ "$mtu" -gt 0 ]
+    if [ -n "$mtu" ] && [ "$mtu" -ge 68 ]
     then
         log debug "setting $dev MTU to $mtu"
         ip link set dev ${dev} mtu ${mtu} || :