Formally disallow dm_accept_partial_bio() on clones created by
__send_duplicate_bios() because their len_ptr points to a shared
unsigned int.  __send_duplicate_bios() is only used for flush bios
and other "abnormal" bios (discards, writezeroes, etc). And
dm_accept_partial_bio() already didn't support flush bios.

Also refactor __send_changing_extent_only() to reflect it cannot fail.
As such __send_changing_extent_only() can update the clone_info before
__send_duplicate_bios() is called to fan-out __map_bio() calls.
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>