crypto: x86/curve25519 - fix cpu feature checking logic in mod_exit

In curve25519_mod_init() the curve25519_alg will be registered only when
(X86_FEATURE_BMI2 && X86_FEATURE_ADX). But in curve25519_mod_exit()
it still checks (X86_FEATURE_BMI2 || X86_FEATURE_ADX) when do crypto
unregister. This will trigger a BUG_ON in crypto_unregister_alg() as
alg->cra_refcnt is 0 if the cpu only supports one of X86_FEATURE_BMI2
and X86_FEATURE_ADX.

Fixes: 07b586fe ("crypto: x86/curve25519 - replace with formally verified implementation")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/x86/crypto/curve25519-x86_64.c

@@ -1500,7 +1500,7 @@ static int __init curve25519_mod_init(void)
 static void __exit curve25519_mod_exit(void)
 {
 	if (IS_REACHABLE(CONFIG_CRYPTO_KPP) &&
-	    (boot_cpu_has(X86_FEATURE_BMI2) || boot_cpu_has(X86_FEATURE_ADX)))
+	    static_branch_likely(&curve25519_use_bmi2_adx))
 		crypto_unregister_kpp(&curve25519_alg);
 }