kernel: remove PF_FLUSHER

PF_FLUSHER is only ever set, not tested, remove it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --cc fs/fs-writeback.c
Simple merge

diff --cc include/linux/sched.h
Simple merge

diff --cc mm/backing-dev.c
Simple merge