mm/writeback: commit reason of WB_REASON_FORKER_THREAD mismatch name

After commit 839a8e86 ("writeback: replace custom worker pool
implementation with unbound workqueue"), there is no bdi forker thread
any more.  However, WB_REASON_FORKER_THREAD is still used due to it is
TPs userland visible and we won't be exposing exactly the same
information with just a different name.
Signed-off-by: NWanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/writeback.h

@@ -47,6 +47,12 @@ enum wb_reason {
 	WB_REASON_LAPTOP_TIMER,
 	WB_REASON_FREE_MORE_MEM,
 	WB_REASON_FS_FREE_SPACE,
+	/*
+	 * There is no bdi forker thread any more and works are done
+	 * by emergency worker, however, this is TPs userland visible
+	 * and we'll be exposing exactly the same information,
+	 * so it has a mismatch name.
+	 */
 	WB_REASON_FORKER_THREAD,
 
 	WB_REASON_MAX,