tsnep: Add XDP socket zero-copy TX support

Send and complete XSK pool frames within TX NAPI context. NAPI context
is triggered by ndo_xsk_wakeup.

Test results with A53 1.2GHz:

xdpsock txonly copy mode, 64 byte frames:
                   pps            pkts           1.00
tx                 284,409        11,398,144
Two CPUs with 100% and 10% utilization.

xdpsock txonly zero-copy mode, 64 byte frames:
                   pps            pkts           1.00
tx                 511,929        5,890,368
Two CPUs with 100% and 1% utilization.

xdpsock l2fwd copy mode, 64 byte frames:
                   pps            pkts           1.00
rx                 248,985        7,315,885
tx                 248,921        7,315,885
Two CPUs with 100% and 10% utilization.

xdpsock l2fwd zero-copy mode, 64 byte frames:
                   pps            pkts           1.00
rx                 254,735        3,039,456
tx                 254,735        3,039,456
Two CPUs with 100% and 4% utilization.

Packet rate increases and CPU utilization is reduced in both cases.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>