crypto/qat: fix crashes

crypto/qat: fix crashes

Resolution for seg-faults observed:
1) in buffer re-alignment in-place sgl case
2) case where data end is exactly at end of an sgl segment.
Also renamed variable and increased comments for clearer code.

Fixes: 40002f6c2a24 ("crypto/qat: extend support for digest-encrypted auth-cipher")
Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Damian Nowak <damianx.nowak@intel.com>