crypto: rng - Free default RNG on module exit

When the rng module is removed the default RNG will be leaked.
Call crypto_del_default_rng to free it if possible.

Fixes: 7cecadb7cca8 ("crypto: rng - Do not free default RNG when it becomes unused")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>