crypto: geniv - convert to new way of freeing instances

Convert the "seqiv" template to the new way of freeing instances where a
->free() method is installed to the instance struct itself.  Also remove
the unused implementation of the old way of freeing instances from the
"echainiv" template, since it's already using the new way too.

In doing this, also simplify the code by making the helper function
aead_geniv_alloc() install the ->free() method, instead of making seqiv
and echainiv do this themselves.  This is analogous to how
skcipher_alloc_instance_simple() works.

This will allow removing support for the old way of freeing instances.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>