This patch changes crypto_grab_spawn to retain the reference count
on the algorithm.  This is because the caller needs to access the
algorithm parameters and without the reference count the algorithm
can be freed at any time.

The reference count will be subsequently dropped by the crypto API
once the instance has been registered.  The helper crypto_drop_spawn
will also conditionally drop the reference count depending on whether
it has been registered.

Note that the code is actually added to crypto_init_spawn.  However,
unless the caller activates this by setting spawn->dropref beforehand
then nothing happens.  The only caller that sets dropref is currently
crypto_grab_spawn.

Once all legacy users of crypto_init_spawn disappear, then we can
kill the dropref flag.

Internally each instance will maintain a list of its spawns prior
to registration.  This memory used by this list is shared with
other fields that are only used after registration.  In order for
this to work a new flag spawn->registered is added to indicate
whether spawn->inst can be used.

Fixes: d6ef2f19 ("crypto: api - Add crypto_grab_spawn primitive")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>