app/testpmd: fix timestamp init in txonly mode

app/testpmd: fix timestamp init in txonly mode

The testpmd application forwards data in multiple threads.
In the txonly mode the Tx timestamps must be initialized
on per thread basis to provide phase shift for the packet
burst being sent. This per thread initialization was performed
on zero value of the variable in thread local storage and
happened only once after testpmd forwarding start. Executing
"start" and "stop" commands did not cause thread local variables
zeroing and wrong timestamp values were used.

Fixes: 4940344dab1d ("app/testpmd: add Tx scheduling command")

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>