ice: refactor ICE_TC_FLWR_FIELD_ENC_OPTS

FLOW_DISSECTOR_KEY_ENC_OPTS can be used for multiple headers, but currently
it is treated as GTP-exclusive in ice. Rename ICE_TC_FLWR_FIELD_ENC_OPTS to
ICE_TC_FLWR_FIELD_GTP_OPTS and check for tunnel type earlier. After this
refactor, it is easier to add new headers using FLOW_DISSECTOR_KEY_ENC_OPTS
- instead of checking tunnel type in ice_tc_count_lkups() and
ice_tc_fill_tunnel_outer(), it needs to be checked only once, in
ice_parse_tunnel_attr().

Signed-off-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>