vxlan: silence one build warning

vxlan: silence one build warning

drivers/net/vxlan.c: In function ‘vxlan_sock_add’:
drivers/net/vxlan.c:2298:11: warning: ‘sock’ may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/net/vxlan.c:2275:17: note: ‘sock’ was declared here
  LD      drivers/net/built-in.o

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>