dm vdo: don't refer to dedupe_context after releasing it

Clear the dedupe_context pointer in a data_vio whenever ownership of
the context is lost, so that vdo can't examine it accidentally.

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