Replace internal sg_count() function and the convoluted logic
around it with the standard sg_nents_for_len() function.
src_nents, dst_nents now hold the number of SW S/G entries,
instead of the HW S/G table entries.

With this change, null (zero length) input data for AEAD case
needs to be handled in a visible way. req->src is no longer
(un)mapped, pointer address is set to 0 in SEQ IN PTR command.
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>