dm vdo vio-pool: add a pool pointer to pooled_vio

This allows us to simplify the return_vio_to_pool interface.

Also, we don't need to use vdo_forget on local variables or arguments
that are about to go out of scope anyway.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>