net: add basic ping/pong type workload support

net: add basic ping/pong type workload support

If you use ioengine=net and then set pingpong=1, fio will operate
in a ping pong type fashion. When the reader receives a package
from the writer, it will return the package to the writer again.
The writer, in turn, will wait for this package before writing
a new package. This makes fio useful for roundrobin measurements
on networks.

The submission latency then measures how long it took to submit
a packet to the local system, while the completion latency is how
long it takes for the package to be received and returned by
the other end.

Signed-off-by: Jens Axboe <axboe@kernel.dk>