btrfs: use btrfs_inode in extent_writepage()

[ Upstream commit 011a9a1f ]

As extent_writepage() is internal helper we should use our inode type,
so change it from struct inode.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 8bf334be ("btrfs: fix double accounting race when extent_writepage_io() failed")
Signed-off-by: Sasha Levin <sashal@kernel.org>