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.

Approved by: so
Security: EN-21:14.pms
MFC after: 1 week
Sponsored by: iXsystems, Inc.

(cherry picked from commit 3e347834200b5d91a33384e696793e4ac20a44d4)
(cherry picked from commit 6514cb18d94e9ab46406b7077d207c8b1a551e6e)