selftests/xsk: submit correct number of frames in populate_fill_ring

selftests/xsk: submit correct number of frames in populate_fill_ring

Submit the correct number of frames in the function
xsk_populate_fill_ring(). For the tests that set the flag
use_addr_for_fill, uninitialized buffers were sent to the fill ring
following the correct ones. This has no impact on the tests, since
they only use the ones that were initialized. But for correctness,
this should be fixed.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-4-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>