buffer: switch do_emergency_thaw() away from pdflush_operation()

This is (again) a preparatory patch similar to commit
a2a9537a

. It open codes a simple
async way of executing do_thaw_all() out of context, so we can get
rid of pdflush.

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