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: Dan Carpenter <error27@gmail.com>
Signed-off-by: Joern Engel <joern@logfs.org>
diff --git a/fs/logfs/dev_mtd.c b/fs/logfs/dev_mtd.c
index cafb6ef..b02a402 100644
--- a/fs/logfs/dev_mtd.c
+++ b/fs/logfs/dev_mtd.c
@@ -250,5 +250,7 @@
 	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);
 }