_buf_ is an array and the one that must be freed is _tp_ instead.

Fixes: a870a02c ("pktgen: use dynamic allocation for debug print buffer")
Reported-by: NWang Jian <jianjian.wang1@gmail.com>
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>