selftests/xsk: Fix for SEND_RECEIVE_UNALIGNED test

Fix test broken by shared umem test and framework enhancement commit.

Correct the current implementation of pkt_stream_replace_half() by
ensuring that nb_valid_entries are not set to half, as this is not true
for all the tests. Ensure that the expected value for valid_entries for
the SEND_RECEIVE_UNALIGNED test equals the total number of packets sent,
which is 4096.

Create a new function called pkt_stream_pkt_set() that allows for packet
modification to meet specific requirements while ensuring the accurate
maintenance of the valid packet count to prevent inconsistencies in packet
tracking.

Fixes: 6d198a89

 ("selftests/xsk: Add a test for shared umem feature")
Reported-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20231214130007.33281-1-tushar.vyavahare@intel.com