mtd: introduce mtd_read_oob interface

drivers/mtd/inftlcore.c

drivers/mtd/mtdchar.c

drivers/mtd/mtdconcat.c

drivers/mtd/mtdpart.c

drivers/mtd/mtdswap.c

drivers/mtd/nand/nand_bbt.c

drivers/mtd/nftlcore.c

drivers/mtd/sm_ftl.c

drivers/mtd/ssfdc.c

drivers/mtd/tests/mtd_oobtest.c

drivers/mtd/tests/mtd_readtest.c

drivers/staging/spectra/lld_mtd.c

fs/jffs2/wbuf.c

include/linux/mtd/mtd.h