We recently started trying to merge the jffs2_raw_node_ref structures
for adjacent obsolete nodes. Fix a bunch of problems in that:
 - Free inode cache structure for a dead inode which is now completely gone.
 - Fix races vs. erase code which also walks the same lists.
 - Fix BUG() when trying to merge with a node which still belongs to an inode.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>