vxlan_core: fix build warnings in vxlan_xmit_one

Fix the below build warnings reported by kernel test robot:
   - initialize vni in vxlan_xmit_one
   - wrap label in ipv6 enabled checks in vxlan_xmit_one

warnings:
static
   drivers/net/vxlan/vxlan_core.c:2437:14: warning: variable 'label' set
but not used [-Wunused-but-set-variable]
           __be32 vni, label;
                       ^

>> drivers/net/vxlan/vxlan_core.c:2483:7: warning: variable 'vni' is
used uninitialized whenever 'if' condition is true
[-Wsometimes-uninitialized]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>