logfs: handle errors from get_mtd_device()

The get_mtd_device() function returns error pointers on failure and if we
don't handle it, it leads to a crash.
Signed-off-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NJoern Engel <joern@logfs.org>

fs/logfs/dev_mtd.c

@@ -250,5 +250,7 @@ int logfs_get_sb_mtd(struct file_system_type *type, int flags,
 	const struct logfs_device_ops *devops = &mtd_devops;
 
 	mtd = get_mtd_device(NULL, mtdnr);
+	if (IS_ERR(mtd))
+		return PTR_ERR(mtd);
 	return logfs_get_sb_device(type, flags, mtd, NULL, devops, mnt);
 }