crypto/qat: optimise check for chained mbufs

crypto/qat: optimise check for chained mbufs

To detect if sgl, use nb_segs > 1, instead of checking for next pointer,
as nb_segs is in first cache-line while next is in second cache-line.

Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Acked-by: Arek Kusztal <arkadiuszx.kusztal@intel.com>