Set interrupt-unmask flag for the drive.

Enabled the driver to unmask other interrupts during processing of a disk interrupt, which greatly improves Linux's responsiveness and eliminates "serial port overrun" errors.

Use this feature with caution: some drive/controller combinations do not tolerate the increased I/O latencies possible when this feature is enabled, resulting in massive filesystem corruption.
In particular CMD-640B and RZ1000 (E)IDE interfaces can be unreliable (due to a hardware flaw) when this option is used with kernel versions earlier than 2.0.13.