Remove isa_unchecked_dma and some more GFP_DMAs in the mid layer

This patchkit fixes all existing drivers that used isa_unchecked_dma to not need that anymore. I have some upcoming infrastructure changes for DMA memory management and isa_unchecked_dma was in the way.

Enabling isa_unchecked_dma had several effects:
- All incoming scsi_cmnds were in GFP_DMA memory.
Only one driver relied on that actually (advansys), the others all accessed the scsi_cmnds only with the CPU.
- scsi hostdata is allocated with GFP_DMA
A lot of drivers relied on that. I converted them all to allocate hostdata in a separate buffer linked to rom the scsi host structure.
- Enabling block layer bouncing for all data. That's the most important one. I changed all drivers to do that directly instead of relying on the mid layer for it.
- sense_buffer is allocated with GFP_DMA. That was also commonly required and not easy to fix so I created a separate host template field that enables sense_buffer bouncing.

Also while I was it I removed also a lot of GFP_DMAs in the frontend drivers which are not needed anymore because the block layer does the bouncing for all data anyways.

The main problem of the patchkit is that is that I wasn't able to test the drivers because I don't have any of the hardware. All changes (except perhaps advansys) were relatively simple and straight forward so I don't expect many problems though.

If anybody has any of these ISA SCSI adapters and would be willing to test them with these patches that would be appreciated.
I suspect actually that some of the ISA drivers are actually already bitrotted independently of these changes. Hopefully they won't make anything worse though.

Patches against 2.6.25rc2

These are a lot of patches. I can set up a git tree if that makes merging easier. Please let me now if I should do that.

-Andi