fix unbalanced wb->list_lock in 3.5-rc1

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Merge tag 'writeback-lock-fix' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux

Pull writeback locking fix from Wu Fengguang:
 "fix unbalanced wb->list_lock in 3.5-rc1"

* tag 'writeback-lock-fix' of git://git.kernel.org/pub/scm/linux/kernel/git/wfg/linux:
  writeback: Fix lock imbalance in writeback_sb_inodes()

fs/fs-writeback.c

@@ -664,6 +664,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 			/* Wait for I_SYNC. This function drops i_lock... */
 			inode_sleep_on_writeback(inode);
 			/* Inode may be gone, start again */
+			spin_lock(&wb->list_lock);
 			continue;
 		}
 		inode->i_state |= I_SYNC;