libnvdimm, namespace: remove redundant initialization of 'nd_mapping'

Pointer nd_mapping is being initialized to a value that is never read,
instead it is being updated to a new value in all the cases where it
is being read afterwards, hence the initialization is redundant and
can be removed.

Cleans up clang warning:
drivers/nvdimm/namespace_devs.c:2411:21: warning: Value stored to
'nd_mapping' during its initialization is never rea

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>