Mostly revert r260267 and hopefully really fix the original problem.

The latest draft of SBC-3 tells: "A MAXIMUM UNMAP LBA COUNT field set to
a non-zero value indicates the maximum number of LBAs that may be unmapped
by an UNMAP command."  To me it does not sound like that limit is set per
single descriptor, but rather per all command.  And I have at least one
device that behaves exactly that way.  This patch fixes the problem there.