Call pipeselwakeup() after toggling PIPE_EOF.

This ensures that pipe_poll() and the pipe kqueue filters observe
PIPE_EOF and set EV_EOF accordingly. As a result an extra call to
knote() after setting PIPE_EOF is unnecessary.

Submitted by: Jan Kokemüller <jan.kokemueller@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24528