[JFFS2] Fix refile of blocks due to write failure.

avoid segfault when nextblock was refiled because of a write failure
- avoid filing blocks on the clean list when they have wasted
space

Signed-off-by: Estelle Hammache <estelle.hammache@st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>