nvme: limit namespace character-device I/O size

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

Reviewed by: ngie, imp
Differential Revision: https://reviews.freebsd.org/D59152