[MTD] return error code from get_mtd_device()

get_mtd_device() returns NULL in case of any failure. Teach it to return an
error code instead. Fix all users as well.

Signed-off-by: Artem Bityutskiy <dedekind@infradead.org>
diff --git a/fs/jffs/jffs_fm.c b/fs/jffs/jffs_fm.c
index 29b68d9..6aab317 100644
--- a/fs/jffs/jffs_fm.c
+++ b/fs/jffs/jffs_fm.c
@@ -17,6 +17,7 @@
  *
  */
 #include <linux/slab.h>
+#include <linux/err.h>
 #include <linux/blkdev.h>
 #include <linux/jffs.h>
 #include "jffs_fm.h"
@@ -104,7 +105,7 @@
 
 	mtd = get_mtd_device(NULL, unit);
 
-	if (!mtd) {
+	if (IS_ERR(mtd)) {
 		kfree(fmc);
 		DJM(no_jffs_fmcontrol--);
 		return NULL;