crypto: talitos - Add a helper function to clear j_extent field

j_extent field is specific to SEC2 so we add a helper function to clear it
so that SEC1 can redefine that function as nop

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>