lib/mpi: mpi_read_raw_from_sgl(): replace len argument by nbytes

Currently, the nbytes local variable is calculated from the len argument
as follows:

  ... mpi_read_raw_from_sgl(..., unsigned int len)
  {
    unsigned nbytes;
    ...
    if (!ents)
      nbytes = 0;
    else
      nbytes = len - lzeros;
    ...
  }

Given that nbytes is derived from len in a trivial way and that the len
argument is shadowed by a local len variable in several loops, this is just
confusing.

Rename the len argument to nbytes and get rid of the nbytes local variable.
Do the nbytes calculation in place.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>