writeback: fix bad _bh spinlock nesting

Fix a bug where a lock is _bh nested within another _bh lock,
but forgets to use the _bh variant for unlock.

Further more, it's not necessary to test _bh locks, the inner lock
can just use spin_lock(). So fix up the bug by making that change.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

mm/backing-dev.c

@@ -415,7 +415,8 @@ static int bdi_forker_thread(void *ptr)
 				break;
 			}
 
-			spin_lock_bh(&bdi->wb_lock);
+			spin_lock(&bdi->wb_lock);
+
 			/*
 			 * If there is no work to do and the bdi thread was
 			 * inactive long enough - kill it. The wb_lock is taken
@@ -432,7 +433,7 @@ static int bdi_forker_thread(void *ptr)
 				action = KILL_THREAD;
 				break;
 			}
-			spin_unlock_bh(&bdi->wb_lock);
+			spin_unlock(&bdi->wb_lock);
 		}
 		spin_unlock_bh(&bdi_lock);