app/testpmd: remove restriction on Tx segments set

app/testpmd: remove restriction on Tx segments set

Currently, if nb_txd is not set, the txpkts is not allowed to be set
because the nb_txd is used to avoid the number of segments exceed the Tx
ring size and the default value of nb_txd is 0. And there is a bug that
nb_txd is the global configuration for Tx ring size and the ring size
could be changed by some command per queue. So these valid check is
unreliable and introduced unnecessary constraints.

This patch adds a valid check function to use the real Tx ring size to
check the validity of txpkts.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>