btrfs: use ASSERT to report logical error in cow_file_range()

Use ASSERT to report logical error in cow_file_range(), also move it a
bit closer to when the num_bytes is derived.

The extent start could be (u64)-1 in some cases, the assert should catch
that we do not accidentally pass it to cow_file_range.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>