SEC1 doesn't support S/G in descriptors so for hash operations,
the CPU has to build a buffer containing the buffered block and
the incoming data. This generates a lot of memory copies which
represents more than 50% of CPU time of a md5sum operation as
shown below with a 'perf record'.

|--86.24%-- kcapi_md_digest
|          |
|          |--86.18%-- _kcapi_common_vmsplice_chunk_fd
|          |          |
|          |          |--83.68%-- splice
|          |          |          |
|          |          |          |--83.59%-- ret_from_syscall
|          |          |          |          |
|          |          |          |          |--83.52%-- sys_splice
|          |          |          |          |          |
|          |          |          |          |          |--83.49%-- splice_from_pipe
|          |          |          |          |          |          |
|          |          |          |          |          |          |--83.04%-- __splice_from_pipe
|          |          |          |          |          |          |          |
|          |          |          |          |          |          |          |--80.67%-- pipe_to_sendpage
|          |          |          |          |          |          |          |          |
|          |          |          |          |          |          |          |          |--78.25%-- hash_sendpage
|          |          |          |          |          |          |          |          |          |
|          |          |          |          |          |          |          |          |          |--60.08%-- ahash_process_req
|          |          |          |          |          |          |          |          |          |          |
|          |          |          |          |          |          |          |          |          |          |--56.36%-- sg_copy_buffer
|          |          |          |          |          |          |          |          |          |          |          |
|          |          |          |          |          |          |          |          |          |          |          |--55.29%-- memcpy
|          |          |          |          |          |          |          |          |          |          |          |

However, unlike SEC2+, SEC1 offers the possibility to chain
descriptors. It is therefore possible to build a first descriptor
pointing to the buffered data and a second descriptor pointing to
the incoming data, hence avoiding the memory copy to a single
buffer.

With this patch, the time necessary for a md5sum on a 90Mbytes file
is approximately 3 seconds. Without the patch it takes 6 seconds.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>