linux: __get_nprocs_sched: do not feed CPU_COUNT_S with garbage [BZ #28850]

Pass the actual number of bytes returned by the kernel.

Fixes: 33099d72 ("linux: Simplify get_nprocs")
Reviewed-by: Dmitry V. Levin <ldv@altlinux.org>

(cherry picked from commit 97ba273b)