crypto: skcipher - use the new scatterwalk functions

Convert skcipher_walk to use the new scatterwalk functions.

This includes a few changes to exactly where the different parts of the
iteration happen.  For example the dcache flush that previously happened
in scatterwalk_done() now happens in scatterwalk_dst_done() or in
memcpy_to_scatterwalk().  Advancing to the next sg entry now happens
just-in-time in scatterwalk_clamp() instead of in scatterwalk_done().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>