[JFFS2] Call summary collector for all mtd devices with writev support

Do the summary collection in the right place. If the device
was not writebuffered but had c->mtd->writev function
(e.g. blkmtd) the summary collector function was not called.

Signed-off-by: Ferenc Havasi <havasi@inf.u-szeged.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>