libnvdimm: Fix endian conversion issues 

nd_label->dpa issue was observed when trying to enable the namespace created
with little-endian kernel on a big-endian kernel. That made me run
`sparse` on the rest of the code and other changes are the result of that.

Fixes: d9b83c75 ("libnvdimm, btt: rework error clearing")
Fixes: 9dedc73a ("libnvdimm/btt: Fix LBA masking during 'free list' population")
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/20190809074726.27815-1-aneesh.kumar@linux.ibm.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>