f2fs: support discard submission error injection

This patch adds to support discard submission error injection for testing
error handling of __submit_discard_cmd().

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index e5c518b..367e979 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -58,6 +58,7 @@
 	FAULT_TRUNCATE,
 	FAULT_IO,
 	FAULT_CHECKPOINT,
+	FAULT_DISCARD,
 	FAULT_MAX,
 };