writeback: check for registered bdi in flusher add and inode dirty

Also a debugging aid. We want to catch dirty inodes being added to
backing devices that don't do writeback.

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

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

diff --cc include/linux/backing-dev.h
Simple merge

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