odhcp6c : Silence mtu write error warnings

Silence warning "daemon.notice netifd: wan6 (1139): sh: write error: Invalid argument"
when an invalid MTU is received via RA as kernel refuses to accept IPv6 mtu values
which are smaller than 1280 and bigger than the device mtu.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn+ssh://svn.openwrt.org/openwrt/trunk@49054 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/network/ipv6/odhcp6c/files/dhcpv6.script

@@ -185,7 +185,7 @@ setup_interface () {
 	# Apply IPv6 / ND configuration
 	HOPLIMIT=$(cat /proc/sys/net/ipv6/conf/$device/hop_limit)
 	[ -n "$RA_HOPLIMIT" -a -n "$HOPLIMIT" ] && [ "$RA_HOPLIMIT" -gt "$HOPLIMIT" ] && echo "$RA_HOPLIMIT" > /proc/sys/net/ipv6/conf/$device/hop_limit
-	[ -n "$RA_MTU" ] && [ "$RA_MTU" -gt 0 ] && echo "$RA_MTU" > /proc/sys/net/ipv6/conf/$device/mtu
+	[ -n "$RA_MTU" ] && [ "$RA_MTU" -ge 1280 ] && echo "$RA_MTU" > /proc/sys/net/ipv6/conf/$device/mtu 2>/dev/null
 	[ -n "$RA_REACHABLE" ] && [ "$RA_REACHABLE" -gt 0 ] && echo "$RA_REACHABLE" > /proc/sys/net/ipv6/neigh/$device/base_reachable_time_ms
 	[ -n "$RA_RETRANSMIT" ] && [ "$RA_RETRANSMIT" -gt 0 ] && echo "$RA_RETRANSMIT" > /proc/sys/net/ipv6/neigh/$device/retrans_time_ms