Extend qat_bl_sgl_to_bufl() to allow skipping the mapping of a region
of the source and the destination scatter lists starting from byte
zero.

This is to support the ZLIB format (RFC 1950) in the qat driver.
The ZLIB format is made of deflate compressed data surrounded by a
header and a footer. The QAT accelerators support only the deflate
algorithm, therefore the header should not be mapped since it is
inserted in software.
Signed-off-by: Lucas Segarra Fernandez <lucas.segarra.fernandez@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>