Retire devctl_notify_f()

devctl_notify_f isn't needed, so retire it. The flags argument is now
unused, so rather than keep it around, retire it. Convert all old
users of it to devctl_notify(). This path no longer sleeps, so is safe
to call from any context. Since it doesn't sleep, it doesn't need to
know if it is OK to sleep or not.

Reviewed by: markj@
Differential Revision: https://reviews.freebsd.org/D26140