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/hugetlbfs/inode.c

@@ -374,7 +374,7 @@ static void truncate_hugepages(struct inode *inode, loff_t lstart)
 static void hugetlbfs_evict_inode(struct inode *inode)
 {
 	truncate_hugepages(inode, 0);
-	clear_inode(inode);
+	end_writeback(inode);
 }
 
 static inline void