vxlan: correctly validate VXLAN ID against VXLAN_N_VID

commit 4e37d6911f36545b286d15073f6f2222f840e81c upstream.

The incorrect check caused an off-by-one error: the maximum VID 0xffffff
was unusable.

Fixes: d342894c ("vxlan: virtual extensible lan")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>