app/testpmd: fix txonly forwarding

app/testpmd: fix txonly forwarding

When random number of Tx segments is enabled, because the actual
number of segments may be only one, the first segment of the Tx
packets must accommodate a complete being sending Eth/IP/UDP packet.

Besides, if multiple flow is enabled, the forwarding will update
the IP and UDP header, these headers shouldn't cross segments.
This also requires the first Tx segment can accommodate a complete
Eth/IP/UDP packet.

In addition, if time stamp is enabled, the forwarding needs more
Tx segment space for time stamp information.

This patch adds checks in beginning of forward engine to make sure
all above conditions are met.

Bugzilla ID: 797
Fixes: 79bec05b32b7 ("app/testpmd: add ability to split outgoing packets")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>