char: Mark /dev/zero and /dev/kmem as not capable of writeback

char: Mark /dev/zero and /dev/kmem as not capable of writeback

commit 371d217ee1ff8b418b8f73fb2a34990f951ec2d4 upstream.

These devices don't do any writeback but their device inodes still can get
dirty so mark bdi appropriately so that bdi code does the right thing and files
inodes to lists of bdi carrying the device inodes.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>