nvme: limit namespace character-device I/O size

[seuros]

Summary

The namespace character device does not initialize si_iosize_max, so
physio falls back to DFLTPHYS and can produce a bio larger than the
qpair payload DMA tag on a controller whose maximum transfer size is
below 64KB. Such a bio fails DMA mapping and is never submitted.

Approved by: ngie (co-mentor)
MFC after: 1 week

[seuros]

rebase

[ngie]

Could this be reproed using nvmecontrol reset? How can one confirm that the new value is being set properly? Asking as part of a "brain-storm" for a regression test of some kind.
Approved by: ngie (co-mentor)

[imp]

sys/dev/nvme/nvme_ns.c

606

A comment here about why we need to update this after a reset would be nice. Why would it change?