jffs2: use cond_resched() instead of yield()

yield() has different semantics meanwhile and even causes RT-kernels to
BUG. Replace the only appearance left in jffs2.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>