jffs2: call jffs2_write_super from jffs2_sync_fs

The call to ->write_super from __sync_filesystem will go away, so make
sure jffs2 performs the same actions from inside ->sync_fs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>