Adds support for writing multiple sectors at once. This allows
back-to-back transfers of sectors giving roughly double write throughput.

To be able to detect which sector is causing problems the system falls
back to single sector writes if a failure is detected.

Tested by several people with no side-effects found.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

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Previously submitted: 2005-03-16
Previously submitted: 2004-12-03