[NET_SCHED]: em_meta: fix compile warning

net/sched/em_meta.c: In function 'meta_int_vlan_tag':
net/sched/em_meta.c:179: warning: 'tag' may be used uninitialized in this function

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>