vxlan: remove vni zero check and drop for COLLECT_METADATA

This patch drops the vni zero check for COLLECT_METADATA mode.
It is not really needed, vni zero is a valid vni.

Fixes: 3ad7a4b1 ("vxlan: support fdb and learning in COLLECT_METADATA mode"
Reported-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>