crypto: Kconfig - warn about performance overhead of CRYPTO_STATS

Make the help text for CRYPTO_STATS explicitly mention that it reduces
the performance of the crypto API.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>