tsnep: Fix mapping for zero copy XDP_TX action

For XDP_TX action xdp_buff is converted to xdp_frame. The conversion is
done by xdp_convert_buff_to_frame(). The memory type of the resulting
xdp_frame depends on the memory type of the xdp_buff. For page pool
based xdp_buff it produces xdp_frame with memory type
MEM_TYPE_PAGE_POOL. For zero copy XSK pool based xdp_buff it produces
xdp_frame with memory type MEM_TYPE_PAGE_ORDER0.

tsnep_xdp_xmit_back() is not prepared for that and uses always the page
pool buffer type TSNEP_TX_TYPE_XDP_TX. This leads to invalid mappings
and the transmission of undefined data.

Improve tsnep_xdp_xmit_back() to use the generic buffer type
TSNEP_TX_TYPE_XDP_NDO for zero copy XDP_TX.

Fixes: 3fc23339

 ("tsnep: Add XDP socket zero-copy RX support")
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>