drbd: nuke pdflush from comments

The pdflush thread is long gone, so this patch removes references to pdflush
from drbd comments.

Cc: drbd-dev@lists.linbit.com
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/block/drbd/drbd_main.c

@@ -3537,9 +3537,9 @@ static void drbd_cleanup(void)
 }
 
 /**
- * drbd_congested() - Callback for pdflush
+ * drbd_congested() - Callback for the flusher thread
  * @congested_data:	User data
- * @bdi_bits:		Bits pdflush is currently interested in
+ * @bdi_bits:		Bits the BDI flusher thread is currently interested in
  *
  * Returns 1<<BDI_async_congested and/or 1<<BDI_sync_congested if we are congested.
  */