vxlan: hide unused local variable

A bugfix introduced a harmless warning in v4.9-rc4:

drivers/net/vxlan.c: In function 'vxlan_group_used':
drivers/net/vxlan.c:947:21: error: unused variable 'sock6' [-Werror=unused-variable]

This hides the variable inside of the same #ifdef that is
around its user. The extraneous initialization is removed
at the same time, it was accidentally introduced in the
same commit.

Fixes: c6fcc4fc ("vxlan: avoid using stale vxlan socket.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>