nvd: set d_delmaxsize to full capacity of NVMe namespace

The NVMe specification has no ability to specify a maximum delete size
that is less than the full capacity of the namespace - so just using the
namespace size is the correct value here.

This fixes reported issues where ZFS trim on init looked like it was
hanging the system - previously the default I/O max size (128KB on
Intel NVMe controllers) was used for delete operations which worked out
to only about 8MB/s.  With this patch I can add an 800GB DC P3700
drive to a ZFS pool in about 15-20 seconds.