[JFFS2] add write verify on dataflash.

Add the write verification buffer to the dataflash.  The mtd_dataflash has
the CONFIG_DATAFLASH_WRITE_VERIFY so is better a change to Kconfig.

Signed-off-by: Michael Trimarchi <trimarchimichael@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>