fs: ubifs: Make k(z)alloc/kfree symmetric

Although kfree() is in fact only a slim wrapper to free() in U-Boot, use
kfree() here, because those structs where allocated with kalloc() or
kzalloc().

Signed-off-by: Alexander Dahl <ada@thorsis.com>
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index bbbbeb5..03ed603 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -1762,8 +1762,8 @@
 	mutex_unlock(&c->umount_mutex);
 	/* Finally free U-Boot's global copy of superblock */
 	if (ubifs_sb != NULL) {
-		free(ubifs_sb->s_fs_info);
-		free(ubifs_sb);
+		kfree(ubifs_sb->s_fs_info);
+		kfree(ubifs_sb);
 		ubifs_sb = NULL;
 	}
 #endif