iomap: Fix use-after-free error in page_done callback

In iomap_write_end, we're not holding a page reference anymore when
calling the page_done callback, but the callback needs that reference to
access the page.  To fix that, move the put_page call in
__generic_write_end into the callers of __generic_write_end.  Then, in
iomap_write_end, put the page after calling the page_done callback.
Reported-by: Jan Kara <jack@suse.cz>
Fixes: 63899c6f ("iomap: add a page_done callback")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/buffer.c

@@ -2104,7 +2104,6 @@ void __generic_write_end(struct inode *inode, loff_t pos, unsigned copied,
 	}
 
 	unlock_page(page);
-	put_page(page);
 
 	if (old_size < pos)
 		pagecache_isize_extended(inode, old_size, pos);
@@ -2160,6 +2159,7 @@ int generic_write_end(struct file *file, struct address_space *mapping,
 {
 	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
 	__generic_write_end(mapping->host, pos, copied, page);
+	put_page(page);
 	return copied;
 }
 EXPORT_SYMBOL(generic_write_end);

fs/iomap.c

@@ -772,6 +772,7 @@ iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
 	__generic_write_end(inode, pos, ret, page);
 	if (iomap->page_done)
 		iomap->page_done(inode, pos, copied, page, iomap);
+	put_page(page);
 
 	if (ret < len)
 		iomap_write_failed(inode, pos, len);