selftests/xsk: Iterate over all the sockets in the send pkts function

Update send_pkts() to handle multiple sockets for sending packets.
Multiple TX sockets are utilized alternately based on the batch size for
improve packet transmission.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20230927135241.2287547-7-tushar.vyavahare@intel.com