lib: os: mpsc_pbuf: take spinlock in utilization getters

Two public functions do not take the spinlock even though
they access mutable, non-atomic members of the buffer:
- mpsc_pbuf_get_utilization()
- mpsc_pbuf_get_max_utilization()

Take the spinlock to avoid possible data races.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>