[JFFS2] Fix slab panic

When JFFS22 is unable to read the root inode, the bad root inode object is not
freed and remains sticked in the jffs2_i slab cache. When we further try to
free the slab cache (e.g., on rmmod jffs2), slab allocator subsystem panics.
Fix this bug.

Signed-off-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>