fix gfs2_stuffed_write_end() on short copies

a) the page is uptodate - ->write_begin() would either fail (in which
case we don't reach ->write_end()), or unstuff the inode, or find the
page already uptodate, or do a successful call of stuffed_readpage(),
which would've made it uptodate

b) zeroing the tail in pagecache is wrong.  kill -9 at the right time
while writing unmodified file contents to the same file should _not_
leave us in a situation when read() from the file will be reporting
it full of zeroes.  Especially since that effect will be transient -
at some later point the page will be evicted and then we'll be back
to the real file contents.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/gfs2/aops.c

@@ -839,12 +839,10 @@ static int gfs2_stuffed_write_end(struct inode *inode, struct buffer_head *dibh,
 	BUG_ON((pos + len) > (dibh->b_size - sizeof(struct gfs2_dinode)));
 	kaddr = kmap_atomic(page);
 	memcpy(buf + pos, kaddr + pos, copied);
-	memset(kaddr + pos + copied, 0, len - copied);
 	flush_dcache_page(page);
 	kunmap_atomic(kaddr);
 
-	if (!PageUptodate(page))
-		SetPageUptodate(page);
+	WARN_ON(!PageUptodate(page));
 	unlock_page(page);
 	put_page(page);