selftests: xsk: Put the same buffer only once in the fill ring

Fix a problem where the fill ring was populated with too many
entries. If number of buffers in the umem was smaller than the fill
ring size, the code used to loop over from the beginning of the umem
and start putting the same buffers in again. This is racy indeed as a
later packet can be received overwriting an earlier one before the Rx
thread manages to validate it.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210922075613.12186-9-magnus.karlsson@gmail.com