This patch adds the hooks cra_init/cra_exit which are called during a tfm's
construction and destruction respectively.  This will be used by the instances
to allocate child tfm's.

For now this lets us get rid of the coa_init/coa_exit functions which are
used for exactly that purpose (unlike the dia_init function which is called
for each transaction).

In fact the coa_exit path is currently buggy as it may get called twice
when an error is encountered during initialisation.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>