[MTD] map_rom has NULL erase pointer

Which means if inftl or similar are loaded with it (which is a dumb thing
to do admittedly) it may oops.

Closes #8108

[dwmw2: change error to -EROFS to match write-protected flash]
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/chips/map_rom.c

@@ -19,6 +19,7 @@ static int maprom_read (struct mtd_info *, loff_t, size_t, size_t *, u_char *);
 static int maprom_write (struct mtd_info *, loff_t, size_t, size_t *, const u_char *);
 static void maprom_nop (struct mtd_info *);
 static struct mtd_info *map_rom_probe(struct map_info *map);
+static int maprom_erase (struct mtd_info *mtd, struct erase_info *info);
 
 static struct mtd_chip_driver maprom_chipdrv = {
 	.probe	= map_rom_probe,
@@ -42,6 +43,7 @@ static struct mtd_info *map_rom_probe(struct map_info *map)
 	mtd->read = maprom_read;
 	mtd->write = maprom_write;
 	mtd->sync = maprom_nop;
+	mtd->erase = maprom_erase;
 	mtd->flags = MTD_CAP_ROM;
 	mtd->erasesize = map->size;
 	mtd->writesize = 1;
@@ -71,6 +73,12 @@ static int maprom_write (struct mtd_info *mtd, loff_t to, size_t len, size_t *re
 	return -EIO;
 }
 
+static int maprom_erase (struct mtd_info *mtd, struct erase_info *info)
+{
+	/* We do our best 8) */
+	return -EROFS;
+}
+
 static int __init map_rom_init(void)
 {
 	register_mtd_chip_driver(&maprom_chipdrv);