Drop READ(6) / WRITE(6) support in da/cd

I'd like to move the default CDB size in the da driver from 6 to 10. I'd
like to make the driver never send READ6 commands (and likely the cd driver
too).

READ10 was an extended command in X.131-1986 (SCSI-1) required for any
computer with self configuring software. It became mandatory in X.131-1994
(SCSI-2), 4 years before CAM entered the tree. It's been required for any
device larger than 2GB. We purposely disable it on all USB and Firewire
attached devices. Its support has been strongly encouraged as an
alternative to READ6 since SBC (1997), with threats to withdraw READ6 once
certain system software had been updated. It became obsolete in SBC-4
(2019).The days of minimizing a couple of bytes in the CDB have long since
passed. We have a lot of quirks to disable these commands for both da and
cd (as well as blanket disabling them for RBC devices).

In short, it's a lot of hassle that we go to, and there doesn't seem like
there's any benefit. A quick search of the mailing list shows the
overwhelming majority of traffic are the problems it causes. I can find no
place where the benefits of using it are explained (though maybe I missed
something).

So, I'd like to remove it before 15, making 10 the minimum CDB for I/O
commands (READ/WRITE,etc). we'll still send 6 byte commands for TEST UNIT
READ, MODE SENSE, etc.

While, as luck would have it, I still have some 100MB and 200MB drives, I
have not HBA that has the right kind of interconnect to read them, so I
can't even experiment to see if these old MAXTOR drives from the 80s
support READ10...

Comments?

Warner