ice: switch: use a struct to pass packet template params

ice_find_dummy_packet() contains a lot of boilerplate code and a
nice room for copy-paste mistakes.
Instead of passing 3 separate pointers back and forth to get packet
template (dummy) params, directly return a structure containing
them. Then, use a macro to compose compound literals and avoid code
duplication on return path.
Now, dummy packet type/name is needed only once to return a full
correct triple pkt-pkt_len-offsets, and those are all one-liners.
dummy_ipv4_gtpu_ipv4_packet_offsets is just moved around and renamed
(as well as dummy_ipv6_gtp_packet_offsets) with no function changes.
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>