[SCSI] convert sg to scsi_execute_async

Convert sg to always send scatterlists, and kill scsi_request usage.

TODO:

- move DIO code to common place or make block layers usable for ULDs.
- move buffer allocation code to common place for all ULDs to use. And
make buffer allocation code obey all queue limits so we can find
out about problems before calling scsi_execute_async. Currently, sg.c
could allocate a buffer that is too large, and send the request
to scsi_execute_async. scsi_execute_async will then check it against
all the queue limits and return a failure in this case. It would nicer
to know about the queue limit violation right away.
- move indirect (copy_to/from_user) paths commone place or make block
layers usable for ULDs.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>