pms(4): Limit maximum I/O size to 256KB instead of 1MB.

There is a weird limit of AGTIAPI_MAX_DMA_SEGS (128) S/G segments per
I/O since the initial driver import. I don't know why it was added,
can only guess some hardware limitation, but in worst case it means
maximum I/O size of 508KB. Respect it to be safe, rounding to 256KB.

MFC after: 1 week
Sponsored by: iXsystems, Inc.