new helper: end_writeback()

Essentially, the minimal variant of ->evict_inode().  It's
a trimmed-down clear_inode(), sans any fs callbacks.  Once
it returns we know that no async writeback will be happening;
every ->evict_inode() instance should do that once and do that
before doing anything ->write_inode() could interfere with
(e.g. freeing the on-disk inode).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -294,6 +294,18 @@ void __iget(struct inode *inode)
 	inodes_stat.nr_unused--;
 }
 
+void end_writeback(struct inode *inode)
+{
+	might_sleep();
+	BUG_ON(inode->i_data.nrpages);
+	BUG_ON(!list_empty(&inode->i_data.private_list));
+	BUG_ON(!(inode->i_state & I_FREEING));
+	BUG_ON(inode->i_state & I_CLEAR);
+	inode_sync_wait(inode);
+	inode->i_state = I_FREEING | I_CLEAR;
+}
+EXPORT_SYMBOL(end_writeback);
+
 /**
  * clear_inode - clear an inode
  * @inode: inode to clear