btrfs: defrag: use btrfs_mod_outstanding_extents in cluster_pages_for_defrag

Since commit 8b62f87b ("Btrfs: rework outstanding_extents"),
manual operations of outstanding_extent in btrfs_inode are replaced by
btrfs_mod_outstanding_extents().
The one in cluster_pages_for_defrag seems to be lost, so replace it
of btrfs_mod_outstanding_extents().

Fixes: 8b62f87b ("Btrfs: rework outstanding_extents")
Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>