add large command support to the block layer

This is an updated patchset for large command support to the block
layer:

http://marc.info/?l=linux-scsi&m=120817127118449&w=2

We rarely handle large commands. So for optimization, a struct request
still has a static array for a command. rq_init sets rq->cmd pointer
to the static array. In short, rq_init() does

rq->cmd = rq->__cmd;

So we can access to rq->cmd and rq->cmd_len as before.

This change requires everyone to initialize the request in a proper
way (that is, just doing a memset() will not work). Now we have
rq_init() that works for any path so this patchset can cleanly convert
users of requests on the stack or kmalloced requests to use it (the
previous patchset does it in a hacky way):

http://marc.info/?l=linux-scsi&m=120911792725876&w=2

This patchset is against Jens' for-linus branch.

#1-#4 patches can be applied via Jens' tree now. #5 patch is for
IDE. It cleanly can be applied to both Bart's latest quilt tree and
Jens' tree though Bart's quilt tree has some pending IDE patches. #4
patch depends on #4. #6 patch depends on #1-#5.

I guess that the easiest way to apply this patchset would be:

1. Pushing Bart's quilt tree to mainline.
2. Rebasing Jens' tree to mainline.
3. Pushing this patchset via Jens' tree.

Jens and Bart, let me know if I can do something to make the process
easier.

Bart, I will try to push the patchset to remove the requests on the
stack for 2.6.27:

http://marc.info/?l=linux-ide&m=120882410712466&w=2


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