selftests: net: fix rps_default_mask with >32 CPUs

If there is more than 32 cpus the bitmask will start to contain
commas, leading to:

./rps_default_mask.sh: line 36: [: 00000000,00000000: integer expression expected

Remove the commas, bash doesn't interpret leading zeroes as oct
so that should be good enough. Switch to bash, Simon reports that
not all shells support this type of substitution.

Fixes: c12e0d5f ("self-tests: introduce self-tests for RPS default mask")
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240122195815.638997-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>