crypto: user - made crypto_user_stat optional

Even if CRYPTO_STATS is set to n, some part of CRYPTO_STATS are
compiled.
This patch made all part of crypto_user_stat uncompiled in that case.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>