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.com

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>