b=5902

b=5902
r=adilger

Many improvements to the code that checksums I/O, among them:
- add a switch to enable or disable them at runtime
- check the pages while under llite's control, and the buffers while under
  the osc/ost's
- if the server and client see different checksums, try to figure out where
  it went wrong
- use the kernel crc32 routine