ioat(4): Add CRC descriptor structure

Add CRC/MOVECRC operations, as well as the TEST and STORE variants.

With these operations, a CRC32C can be computed over one or more
descriptors' source data. When the STORE operation is encountered, the
accumulated CRC32C is emitted to memory. A TEST operations triggers an
IOAT channel error if the accumulated CRC32C does not match one in
memory.

These operations are not exposed through any API yet.

Sponsored by: EMC / Isilon Storage Division