ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

Summary

In the Linux kernel, the following vulnerability has been resolved: ptp: netc: fix period truncation and potential divide-by-zero in PEROUT The max_period bound in net_timer_enable_perout() was computed as: max_period = (u64)NETC_TMR_DEFAULT_FIPER + integral_period; which exceeds U32_MAX when integral_period > 0 (e.g. 0x100000002 for the default 333333333 Hz clock). A period_ns that passes this check but exceeds U32_MAX is then silently truncated when stored into the u32 struct netc_pp::period field. A truncated value of zero can reach netc_timer_set_perout_alarm(), where the local u32 period variable would also be 0, causing a divide-by-zero in roundup_u64(delta, period) whenever the stime < min_time branch is taken (which always happens for a start time of {0, 0}). Additionally, netc_timer_enable_periodic_pulse() and netc_timer_enable_fiper() both compute: fiper = pp->period - integral_period; A zero pp->period results in an unsigned wraparound to 0xFFFFFFFD, mis-programming the FIPER hardware register. Fix all three issues by capping max_period at NETC_TMR_DEFAULT_FIPER (0xFFFFFFFF). This ensures that any period_ns passing the range check fits in a u32 without truncation, so the stored value is always valid and non-zero. The accepted range is reduced by integral_period ns (typically only a few nanoseconds), which is negligible in practice.