With this patch raid5 works.  There is still some more
work to though.

- uses bio instead of buffer_head
- stripe cache is now a fixed size.
   If read requests are smaller, we read the whole block anyway
   If write reqeusts are smaller, we pre-read.
- stripe_head is now variable sized with an array of structures at
  the end.  We allocate extra space depending on how many devices
  are in the array.
  stripe_head has it's very own slab cache.
- store and use bdev for each device in array

by-passing the cache for reads is currently disabled.  I need to
think through the implications (and implementation) of allowing
large bion that are larger than the stripe cache to go directly
to the device (if it isn't failed of-course).