samples/bpf: xdpsock: Add cyclic TX operation capability

samples/bpf: xdpsock: Add cyclic TX operation capability

Tx cycle time is in micro-seconds unit. By combining the batch size (-b M)
and Tx cycle time (-T|--tx-cycle N), xdpsock now can transmit batch-size of
packets every N-us periodically. Cyclic TX operation is not applicable if
--poll mode is used.

To transmit 16 packets every 1ms cycle time for total of 100000 packets
silently:
 $ xdpsock -i eth0 -T -N -z -T 1000 -b 16 -C 100000

To print cyclic TX schedule variance stats, use --app-stats|-a:
 $ xdpsock -i eth0 -T -N -z -T 1000 -b 16 -C 100000 -a

 sock0@eth0:0 txonly xdp-drv
                   pps            pkts           0.00
rx                 0              0
tx                 0              100000

                   calls/s        count
rx empty polls     0              0
fill fail polls    0              0
copy tx sendtos    0              0
tx wakeup sendtos  0              6254
opt polls          0              0

                   period     min        ave        max        cycle
Cyclic TX          1000000    53507      75334      712642     6250

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211230035447.523177-5-boon.leong.ong@intel.com