selftests: drv-net: limit RPS test CPUs to supported range

selftests: drv-net: limit RPS test CPUs to supported range

The _get_unused_cpus() function can return CPU numbers >= 16, which
exceeds RPS_MAX_CPUS in toeplitz.c. When this happens, the test fails
with a cryptic message:

  # Exception| Traceback (most recent call last):
  # Exception|   File "/tmp/cur/linux/tools/testing/selftests/net/lib/py/ksft.py", line 319, in ksft_run
  # Exception|     func(*args)
  # Exception|   File "/tmp/cur/linux/tools/testing/selftests/drivers/net/hw/toeplitz.py", line 189, in test
  # Exception|     with bkg(" ".join(rx_cmd), ksft_ready=True, exit_wait=True) as rx_proc:
  # Exception|          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  # Exception|   File "/tmp/cur/linux/tools/testing/selftests/net/lib/py/utils.py", line 124, in __init__
  # Exception|     super().__init__(comm, background=True,
  # Exception|   File "/tmp/cur/linux/tools/testing/selftests/net/lib/py/utils.py", line 77, in __init__
  # Exception|     raise Exception("Did not receive ready message")
  # Exception| Exception: Did not receive ready message

Rename _get_unused_cpus() to _get_unused_rps_cpus() and cap the CPU
search range to RPS_MAX_CPUS.

Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260210093110.1935149-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>