[JFFS2] Prevent oops after 'node added in wrong place' debug check

jffs2_add_physical_node_ref() should never really return error -- it's
an internal debugging check which triggered. We really need to work out
why and stop it happening. But in the meantime, let's make the failure
mode a little less nasty.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>