libnvdimm, btt: refactor map entry operations with macros

Add helpers for converting a raw map entry to just the block number, or
either of the 'e' or 'z' flags in preparation for actually using the
error flag to mark blocks with media errors.

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