vxlan: add check to prevent use of remote ip attributes with NDA_NH_ID

NDA_NH_ID represents a remote ip or a group of remote ips.
It allows use of nexthop groups in lieu of a remote ip or a
list of remote ips supported by the fdb api.

Current code ignores the other remote ip attrs when NDA_NH_ID is
specified. In the spirit of strict checking, This commit adds a
check to explicitly return an error on incorrect usage.

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>