[JFFS2] Add length argument to jffs2_add_physical_node_ref()

If __totlen is going away, we need to pass the length in separately.
Also stop callers from needlessly setting ref->next_phys to NULL,
since that's done for them... and since that'll also be going away soon.

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