[JFFS2] Fix race in garbage collector

Fix the race problem described here:
http://lists.infradead.org/pipermail/linux-mtd/2005-April/012361.html



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