[JFFS2] Add some preemptive BUG checks for XATTR code

In a couple of places, we assume that what's at the end of the
->next_in_ino list is a struct jffs2_inode_cache. Let's check
for that, since we expect it to change soon.

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