kqueue: drain kqueue taskqueue if syscall tickled it

Otherwise return from the syscall and next syscall, which could be
kevent(2) on the kqueue that should be notified, races with the kqueue
taskqueue thread, and potentially misses the wakeup. This is reliably
visible when kevent(2) only peeks into events using zeroed timeout.

PR: 258310
Reported by: arichardson, Jan Kokemüller <jan.kokemueller@gmail.com>
Reviewed by: arichardson, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31858