Replace update from -r271410 accidentally lost in -r273575.
Add a comment explaining the intent behind the I/O reservation [0x72-0x77].
Allocate buffer for READ BUFFER/WRITE BUFFER commands on demand.

These commands are rare, but consume additional 256KB RAM per LUN.

MFC after:	1 week
Simplify memrw().

MFC after:	10 days