Limit stripesize reported from nvd(4) to 4K

Intel NVMe controllers have a slow path for I/Os that span a 128KB stripe boundary but ZFS limits ashift, which is derived from d_stripesize, to 13 (8KB) so we limit the stripesize reported to geom(8) to 4KB.

This may result in a small number of additional I/Os to require splitting in nvme(4), however the NVMe I/O path is very efficient so these additional I/Os will cause very minimal (if any) difference in performance or CPU utilisation.

This can be controller by the new sysctl kern.nvme.max_optimal_sectorsize.

MFC after: 1 week
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4446