core: Introduce and use cell_owns_cpu

core: Introduce and use cell_owns_cpu

This helper combines the check for the maximum CPU number with the
consultation of the CPU set's bitmap. Both helps to make the code more
compact and avoid to forget the former test in the future.

We keep it inline as it generates quite a bit boilerplate code
otherwise, and IPI transmission depends on efficient execution.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>