mtd: introduce mtd_sync interface

drivers/mtd/ftl.c

drivers/mtd/mtdblock.c

drivers/mtd/mtdchar.c

drivers/mtd/mtdconcat.c

drivers/mtd/mtdpart.c

drivers/mtd/mtdswap.c

drivers/mtd/rfd_ftl.c

drivers/mtd/ubi/kapi.c

fs/jffs2/super.c

fs/logfs/dev_mtd.c

include/linux/mtd/mtd.h