ip: link_gre: Do not send ERSPAN attributes to GRE tunnels

ip: link_gre: Do not send ERSPAN attributes to GRE tunnels

In the commit referenced below, ip link started sending ERSPAN-specific
attributes even for GRE and gretap tunnels. Fix by more carefully
distinguishing between the GRE/tap and ERSPAN modes. Do not show
ERSPAN-related help in GRE/tap mode, likewise do not accept ERSPAN
arguments, or send ERSPAN attributes.

Fixes: 83c543af872e ("erspan: set erspan_ver to 1 by default")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>