mlxsw: spectrum: Don't depend on ip_gre and ip6_gre

mlxsw: spectrum: Don't depend on ip_gre and ip6_gre

mlxsw_spectrum supports offloading of a tc action mirred egress mirror
to a gretap or an ip6gretap netdevice, which necessitates calls to
functions defined in ip_gre, ip6_gre and ip6_tunnel modules. Previously
this was enabled by introducing a hard dependency of MLXSW_SPECTRUM on
NET_IPGRE and IPV6_GRE. However the rest of mlxsw is careful about
picking which modules are absolutely required, and therefore the better
approach is to make mlxsw_spectrum tolerant of absence of one or both of
the GRE flavors.

Hence rework the NET_IPGRE and IPV6_GRE dependencies to just guard
matching modularity, and hide the corresponding code in spectrum_span.c
in an #if IS_ENABLED. Mark mlxsw_sp_span_entry_tunnel_parms_common as
maybe unused, to muffle warnings if neither GRE flavor is selected,
which seems cleaner than introducing a composite #if.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>