We were handling chained scattergather lists with specialized code
needlessly as the regular sg APIs handle them just fine. The code
handling this also had an (unused) code path with a use-before-init
error, flagged by Coverity.

Remove all special handling of chained sg and leave their handling
to the regular sg APIs.
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>