crypto: nx - use the new scatterwalk functions

- In nx_walk_and_build(), use scatterwalk_start_at_pos() instead of a
  more complex way to achieve the same result.

- Also in nx_walk_and_build(), use the new functions scatterwalk_next()
  which consolidates scatterwalk_clamp() and scatterwalk_map(), and use
  scatterwalk_done_src() which consolidates scatterwalk_unmap(),
  scatterwalk_advance(), and scatterwalk_done().  Remove unnecessary
  code that seemed to be intended to advance to the next sg entry, which
  is already handled by the scatterwalk functions.

  Note that nx_walk_and_build() does not actually read or write the
  mapped virtual address, and thus it is misusing the scatter_walk API.
  It really should just access the scatterlist directly.  This patch
  does not try to address this existing issue.

- In nx_gca(), use memcpy_from_sglist() instead of a more complex way to
  achieve the same result.

- In various functions, replace calls to scatterwalk_map_and_copy() with
  memcpy_from_sglist() or memcpy_to_sglist() as appropriate.  Note that
  this eliminates the confusing 'out' argument (which this driver had
  tried to work around by defining the missing constants for it...)

Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N Rao <naveen@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>