[JFFS2] Reduce calls to ref_totlen() in jffs2_mark_node_obsolete()

We were calling ref_totlen() 18 times. Even before that becomes a real
function rather than just a dereference, apparently some compilers still
suck anyway. It'll _certainly_ suck after ref_totlen() becomes more
complicated, so calculate it once and don't rely on CSE.

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