vxlan: fix fdb_dump index calculation

When too many remotes are bound to an FDB entry, index may not be increased.
This problem will be caused on the large scale environment that is based on
the unicast default destination, for instance.

Signed-off-by: Atzm Watanabe <atzm@iij.ad.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>