lwtunnel: fix argument parsing

lwtunnel: fix argument parsing

Currently parse_encap_ip() does not update correctly argv/argc;
if multiple lwtunnel arguments are provided, the parsing fails after
the first one, i.e.

 ip route add 172.16.101.0/24 dev vxlan1 encap ip id 42 dst 192.168.255.1

fails with:

 Error: either "to" is duplicate, or "dst" is a garbage.

This commit addresses the issue, stepping to next argument at each iteration
of the parsing loop.

Fixes: 1e5293056a02 ("lwtunnel: Add encapsulation support to ip route")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>