libnvdimm: fix namespace seed creation

A new BLK namespace "seed" device is created whenever the current seed
is successfully probed.  However, if that namespace is assigned to a BTT
it may never directly experience a successful probe as it is a
subordinate device to a BTT configuration.

The effect of the current code is that no new namespaces can be
instantiated, after the seed namespace, to consume available BLK DPA
capacity.  Fix this by treating a successful BTT probe event as a
successful probe event for the backing namespace.

Reported-by: Nicholas Moulin <nicholas.w.moulin@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>