selftests/xsk: Add packet stream replacement function

Add pkt_stream_replace_ifobject function to replace the packet stream for
a given ifobject.

Enable separate TX and RX packet replacement, allowing RX side packet
length adjustments using bpf_xdp_adjust_tail() in the upcoming patch.
Currently, pkt_stream_replace() works on both TX and RX packet streams,
and this new function provides the ability to modify one of them.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250410033116.173617-2-tushar.vyavahare@intel.com