trace: replace WB_REASON_FOREIGN_FLUSH with a string

Now WB_REASON_FOREIGN_FLUSH is displayed as a number, maybe a
string is better.

v2: replace some space with tab.

Link: https://lkml.kernel.org/r/1619914347-21904-1-git-send-email-brookxu.cn@gmail.comSigned-off-by: Chunguang Xu <brookxu@tencent.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/trace/events/writeback.h

@@ -36,7 +36,8 @@
 	EM( WB_REASON_PERIODIC,			"periodic")		\
 	EM( WB_REASON_LAPTOP_TIMER,		"laptop_timer")		\
 	EM( WB_REASON_FS_FREE_SPACE,		"fs_free_space")	\
-	EMe(WB_REASON_FORKER_THREAD,		"forker_thread")
+	EM( WB_REASON_FORKER_THREAD,		"forker_thread")	\
+	EMe(WB_REASON_FOREIGN_FLUSH,		"foreign_flush")
 
 WB_WORK_REASON