UBI: introduce flash dump helper

Useful for debugging problems, compiled in only if UBI debugging
is enabled. This patch also makes the UBI writing function dump
the flash if it fails to write.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/io.c

@@ -269,6 +269,7 @@ int ubi_io_write(struct ubi_device *ubi, const void *buf, int pnum, int offset,
 		ubi_err("error %d while writing %d bytes to PEB %d:%d, written "
 			"%zd bytes", err, len, pnum, offset, written);
 		ubi_dbg_dump_stack();
+		ubi_dbg_dump_flash(ubi, pnum, offset, len);
 	} else
 		ubi_assert(written == len);