With the new truncate sequence every filesystem that wants to support file
size changes on disk needs to implement its own ->setattr.  So instead
of calling inode_setattr which supports size changes call into a simple
method that doesn't support this.  simple_setattr is almost what we
want except that it does not mark the inode dirty after changes.  Given
that marking the inode dirty is a no-op for the simple in-memory filesystems
that use simple_setattr currently just add the mark_inode_dirty call.

Also add a WARN_ON for the presence of a truncate method to simple_setattr
to catch new instances of it during the transition period.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>