vxlan: allow a user to set TTL value

"ip link add ... type vxlan ... ttl X" allows a user to set the TTL
used by a VXLAN for encapsulation. The provided value was ignored by
vxlan module and the default value of 1 was used when encapsulating
multicast packets.

Signed-off-by: Vincent Bernat <bernat@luffy.cx>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>