[swss] Fix arp_update script (#8412)

Fix #7968

Issue is detected on SONiC.20201231.11

In test_static_route.py::test_static_route_ecmp static routes are configured, but neighbors are not resolved after config reload even after 10 minutes.
It looks like the arp_update script is starting to ping when Vlan1000 is not fully configured.
When issue is reproduced, stuck ping6 process is observed in swss container :

USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         180  0.1  0.0   6296  1272 pts/0    S    17:03   0:03 ping6 -I Vlan1000 -n -q -i 0 -c 1 -W 0 ff02::1
And when arp_update script successfully resolves neighbors, we observe sleep 300 instead of ping process

files/scripts/arp_update

@@ -37,7 +37,7 @@ while /bin/true; do
       eval `eval $ipcmd`
 
       # send ipv6 multicast pings to Vlan interfaces to get/refresh link-local addrs
-      ping6cmd="ping6 -I $vlan -n -q -i 0 -c 1 -W 0 ff02::1 >/dev/null"
+      ping6cmd="timeout 1 ping6 -I $vlan -n -q -i 0 -c 1 -W 0 ff02::1 >/dev/null"
       eval $ping6cmd
 
       # generate a list of ndisc6 commands (exclude link-local addrs since it is done above):