9p: saner ->write_end() on failing copy into non-uptodate page

If we had a short copy into an uptodate page, there's no reason
whatsoever to zero anything; OTOH, if that page had _not_ been
uptodate, we must have been trying to overwrite it completely
and got a short copy.  In that case, overwriting the end with
zeroes, marking uptodate and sending to server is just plain
wrong.  Just unlock, keep it non-uptodate and return 0.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/9p/vfs_addr.c

@@ -309,18 +309,10 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping,
 
 	p9_debug(P9_DEBUG_VFS, "filp %p, mapping %p\n", filp, mapping);
 
-	if (unlikely(copied < len)) {
-		/*
-		 * zero out the rest of the area
-		 */
-		unsigned from = pos & (PAGE_SIZE - 1);
-
-		zero_user(page, from + copied, len - copied);
-		flush_dcache_page(page);
+	if (unlikely(copied < len && !PageUptodate(page))) {
+		copied = 0;
+		goto out;
 	}
-
-	if (!PageUptodate(page))
-		SetPageUptodate(page);
 	/*
 	 * No need to use i_size_read() here, the i_size
 	 * cannot change under us because we hold the i_mutex.
@@ -330,6 +322,7 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping,
 		i_size_write(inode, last_pos);
 	}
 	set_page_dirty(page);
+out:
 	unlock_page(page);
 	put_page(page);