mtd: introduce mtd_write interface

drivers/mtd/chips/cfi_cmdset_0020.c

drivers/mtd/ftl.c

drivers/mtd/mtdblock.c

drivers/mtd/mtdblock_ro.c

drivers/mtd/mtdchar.c

drivers/mtd/mtdconcat.c

drivers/mtd/mtdcore.c

drivers/mtd/mtdoops.c

drivers/mtd/mtdpart.c

drivers/mtd/mtdswap.c

drivers/mtd/rfd_ftl.c

drivers/mtd/tests/mtd_pagetest.c

drivers/mtd/tests/mtd_speedtest.c

drivers/mtd/tests/mtd_stresstest.c

drivers/mtd/tests/mtd_subpagetest.c

drivers/mtd/tests/mtd_torturetest.c

drivers/mtd/ubi/io.c

drivers/staging/spectra/lld_mtd.c

fs/jffs2/wbuf.c

fs/jffs2/writev.c