riscv: hwprobe: Avoid uninitialized read in hwprobe_get_cpus()

riscv: hwprobe: Avoid uninitialized read in hwprobe_get_cpus()

When cpusetsize < cpumask_size(), hwprobe_get_cpus() did not fully
initialize its copy of the cpu mask, which could cause non-deterministic
results from the riscv_hwprobe syscall on a system with more than 8 CPUs
when the supplied cpu mask is empty.  Address this by fully initializing
the cpu mask.

Fixes: e178bf146e4b ("RISC-V: hwprobe: Introduce which-cpus flag")
Signed-off-by: Mark Harris <mark.hsj@gmail.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Michael Ellerman <mpe@kernel.org>
Link: https://patch.msgid.link/20260714003056.73707-1-mark.hsj@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>