convert sg to block layer helpers - v5

There is no big changes between v4 and v5. I was able to fix
things in scsi tgt, so I could remove the weird arguements
the block helpers were taking for it. I also tried to break
up the patchset for easier viewing. The final patch also
takes care of the access_ok regression.

These patches were made against linus's tree since Tomo needed
me to break part of it out for his scsi tgt bug fix patches.

0001-rm-bio-hacks-in-scsi-tgt.txt - Drop scsi tgt's bio_map_user
usage and convert it to blk_rq_map_user. Tomo is also sending
this patch in his patchset since he needs it for his bug fixes.

0002-rm-block-device-arg-from-bio-map-user.txt - The block_device
argument is never used in the bio map user functions, so this
patch drops it.

0003-Support-large-sg-io-segments.txt - Modify the bio functions
to allocate multiple pages at once instead of a single page.

0004-Add-reserve-buffer-for-sg-io.txt - Add reserve buffer support
to the block layer for sg and st indirect IO use.

0005-Add-sg-io-mmap-helper.txt - Add some block layer helpers for
sg mmap support.

0006-Convert-sg-to-block-layer-helpers.txt - Convert sg to block
layer helpers.

0007-mv-user-buffer-copy-access_ok-test-to-block-helper.txt - 
Move user data buffer access_ok tests to block layer helpers.

The goal of this patchset is to remove scsi_execute_async and
reduce code duplication.

People want to discuss further merging sg and bsg/scsi_ioctl
functionality, but I did not handle and any of that in this
patchset since people still disagree on what should supported
with future interfaces.

My only TODO is maybe make the bio reserve buffer mempoolable
(make it work as mempool alloc and free functions). Since
sg only supported one reserve buffer per fd I have not worked
on it and it did not seem worth it if there are no users.


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