libnvdimm, btt: clean up internal interfaces

Consolidate the parameters passed to arena_is_valid into just nd_btt,
and an info block to increase re-usability.

Similarly, btt_arena_write_layout doesn't need to be passed a uuid, as
it can be obtained from arena->nd_btt.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>