vxlan: Fix uninitialized variable warnings.

drivers/net/vxlan.c: In function ‘vxlan_xmit_one’:
drivers/net/vxlan.c:2141:10: warning: ‘err’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: David S. Miller <davem@davemloft.net>