nvd: add hw.nvd.delete_max tunable

The NVMe specification does not define a maximum or optimal delete
size, so technically max delete size is min(full size of namespace,
2^32 - 1 LBAs).  A single delete operation for a multi-TB NVMe
namespace though may take much longer to complete than the nvme(4)
I/O timeout period.  So choose a sensible default here that is still
suitably large to minimize the number of overall delete operations.

This also fixes possible uint32_t overflow on initial TRIM operation
for zpool create operations for NVMe namespaces with >4G LBAs.