selftests/pid_namespace: compute pid_max test limits dynamically

The pid_max kselftest hardcodes pid_max values of 400 and 500, but the
kernel enforces a minimum of PIDS_PER_CPU_MIN * num_possible_cpus().
On machines with many possible CPUs (e.g. nr_cpu_ids=128 yields a
minimum of 1024), writing 400 or 500 to /proc/sys/kernel/pid_max
returns EINVAL and all three tests fail.

Compute these limits the same way as the kernel does and set outer_limit
and inner_limit dynamically based on the result. Original test semantics
are preserved (outer < inner, nested namespace capped by parent).

Signed-off-by: Bjoern Doebel <doebel@amazon.com>
Link: https://patch.msgid.link/20260422201151.3830506-1-doebel@amazon.com
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Assisted-by: Kiro:claude-opus-4.6
Signed-off-by: Christian Brauner <brauner@kernel.org>

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