crypto: scatterwalk - Move skcipher walk and use it for memcpy_sglist

Move the generic part of skcipher walk into scatterwalk, and use
it to implement memcpy_sglist.

This makes memcpy_sglist do the right thing when two distinct SG
lists contain identical subsets (e.g., the AD part of AEAD).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>