net/tls: use the new scatterwalk functions

Replace calls to the deprecated function scatterwalk_copychunks() with
memcpy_from_scatterwalk(), memcpy_to_scatterwalk(), or
scatterwalk_skip() as appropriate.  The new functions generally behave
more as expected and eliminate the need to call scatterwalk_done() or
scatterwalk_pagedone().

However, the new functions intentionally do not advance to the next sg
entry right away, which would have broken chain_to_walk() which is
accessing the fields of struct scatter_walk directly.  To avoid this,
replace chain_to_walk() with scatterwalk_get_sglist() which supports the
needed functionality.

Cc: Boris Pismenny <borisp@nvidia.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>