vxlan: simplify if clause in dev_close

Dan Carpenter's static checker warned that in vxlan_stop we are checking
if 'vs' can be NULL while later we simply derreference it.

As after commit 56ef9c90 ("vxlan: Move socket initialization to
within rtnl scope") 'vs' just cannot be NULL in vxlan_stop() anymore, as
the interface won't go up if the socket initialization fails. So we are
good to just remove the check and make it consistent.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>