filt_fsevent: only record interested events

Respect filter-specific flags for the EVFILT_FS filter.

When a kevent is registered with the EVFILT_FS filter, it is always
triggered when an EVFILT_FS event occurs, regardless of the
filter-specific flags used. Fix that.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D28974