kern/intr: switch intr_event_handle() to return stray count

[ehem_freebsd_m5p.com]

Summary

Comparable to the current situation. Minor savings, one caller simply
passes the count directly through.

[ehem_freebsd_m5p.com]

This works well with D35519/D35529/D35546. The whole stack turns to returning stray counts, which are rather more useful than 0 or EINVAL. Notably for a PPI interrupt, limited reporting of strays can be rather more consistent.