vxlan: fix last fdb index during dump of fdb with nhid

This patch fixes last saved fdb index in fdb dump handler when
handling fdb's with nhid.

Fixes: 1274e1cc

 ("vxlan: ecmp support for mac fdb entries")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>