samples/bpf: xdpsock: Add option to specify number of packets to send

Use '-C' or '--tx-pkt-count' to specify number of packets to send.
If it is not specified, the application sends packets forever. If packet
count is not a multiple of batch size, last batch sent is less than the
batch size.

Signed-off-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191220085530.4980-5-jay.jayatheerthan@intel.com