[JFFS2] Fix NOR only compile

Fix the bug that caouses problems when compiling for NOR.
We read  a newly erased block so we don't need to check ECC.

Define jffs2_is_writebuffered as zero if there is no wbuf.

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