[JFFS2] Handle inodes with only a single metadata node with non-zero isize

[JFFS2] Handle inodes with only a single metadata node with non-zero isize

This should never happen unless there's corruption on the medium and the
actual data nodes go missing. But the failure mode (an oops when we assume
the fragtree isn't empty and go looking for its last node) isn't useful.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>