[JFFS2] Fix unpoint length

Fix a couple of instances in JFFS2 where the unpoint() routine is
being called with the wrong length in cases where the point() routine
truncated a request.

Signed-off-by: Andy Lowe <alowe@mvista.com>
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>