By default, the SCSI mid-layer seems to send down 512KB requests
(sg_tablesize = 256), with some requests occasionally combined. By
allowing the mid-layer to chain requests, we can easily grow to 1024KB
or larger -- I've tested 4096KB I/O requests with no problems.

I looked through the DMA paths on the hardware drivers to ensure they
could take advantage of the SG chaining, and it seems that every one
except ipath uses the system's DMA routines, which have been converted
to handle chaining.  ipath looks like it should be OK, but I have no
way to test it.
Signed-off-by: David Dillow <dillowda@ornl.gov>

[ Tested on ipath.  - Roland ]
Signed-off-by: Roland Dreier <rolandd@cisco.com>