Get rid of pdflush_operation() in emergency sync and remount

Opencode a cheasy approach with kevent. The idea here is that we'll
add some generic delayed work infrastructure, which probably wont be
based on pdflush (or maybe it will, in which case we can just add it
back).

This is in preparation for getting rid of pdflush completely.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>