selftests/xsk: add varying payload pattern within packet

selftests/xsk: add varying payload pattern within packet

Add a varying payload pattern within the packet. Instead of having
just a packet number that is the same for all words in a packet, make
each word different in the packet. The upper 16-bits are set to the
packet number and the lower 16-bits are the sequence number of the
words in this packet. So the 3rd packet's 5th 32-bit word of data will
contain the number (2<<32) | 4 as they are numbered from 0.

This will make it easier to detect fragments that are out of order
when starting to test multi-buffer support.

The member payload in the packet is renamed pkt_nb to reflect that it
is now only a pkt_nb, not the real payload as seen above.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230516103109.3066-4-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>