writeback: fix time ordering of the per superblock dirty inode lists: memory-backed inodes

For reasons which escape me, inodes which are dirty against a ram-backed
filesystem are managed in the same way as inodes which are backed by real
devices.

Probably we could optimise things here.  But given that we skip the entire
supeblock as son as we hit the first dirty inode, there's not a lot to be
gained.

And the code does need to handle one particular non-backed superblock: the
kernel's fake internal superblock which holds all the blockdevs.

Still.  At present when the code encounters an inode which is dirty against a
memory-backed filesystem it will skip that inode by refiling it back onto
s_dirty.  But it fails to update the inode's timestamp when doing so which at
least makes the debugging code upset.

Fix.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>