examples/tep_term: fix inner L4 checksum

examples/tep_term: fix inner L4 checksum

When sending packets from virtual machine which in need of TSO
by hardware NIC, the inner L4 checksum is not correct on the
other side of the cable.

It's because get_psd_sum() depends on PKT_TX_TCP_SEG to calculate
pseudo-header checksum, but currently this bit is set after the
function get_psd_sum() is called. The fix is straightforward.
Move the bit setting before get_psd_sum() is called.

Fixes: a50245ede72a ("examples/tep_term: initialize VXLAN sample")

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>