sg_count() internally calls sg_nents_for_len(), which could fail
in case the required number of bytes is larger than the total
bytes in the S/G.

Thus, add checks to validate the input.
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>