net: ipv6: fix virtual tunneling build

net: ipv6: fix virtual tunneling build

IPv6 PMTUD has been updated to support UID-based routing.

Pass INVALID_UID as the socket UID to ip6_update_pmtu(),
otherwise we run into following build error:
----------------
  CC      net/ipv6/ip6_vti.o
net/ipv6/ip6_vti.c: In function ‘vti6_err’:
net/ipv6/ip6_vti.c:559:3: error: too few arguments to function ‘ip6_update_pmtu’
In file included from include/net/ip_tunnels.h:19:0,
                 from net/ipv6/ip6_vti.c:44:
include/net/ip6_route.h:110:6: note: declared here
make[2]: *** [net/ipv6/ip6_vti.o] Error 1
----------------

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>