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>

crypto/Kconfig

@@ -1393,6 +1393,9 @@ config CRYPTO_STATS
 	help
 	  Enable the gathering of crypto stats.
 
+	  Enabling this option reduces the performance of the crypto API.  It
+	  should only be enabled when there is actually a use case for it.
+
 	  This collects data sizes, numbers of requests, and numbers
 	  of errors processed by:
 	  - AEAD ciphers (encrypt, decrypt)