crypto: af_alg - Decrement struct key.usage in alg_set_by_key_serial()

Calls to lookup_user_key() require a corresponding key_put() to
decrement the usage counter. Once it reaches zero, we schedule key GC.
Therefore decrement struct key.usage in alg_set_by_key_serial().

Fixes: 7984ceb1 ("crypto: af_alg - Support symmetric encryption via keyring keys")
Cc: <stable@vger.kernel.org>
Signed-off-by: Frederick Lawler <fred@cloudflare.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>