lib/crypto: arm/curve25519: Disable on CPU_BIG_ENDIAN

On big endian arm kernels, the arm optimized Curve25519 code produces
incorrect outputs and fails the Curve25519 test.  This has been true
ever since this code was added.

It seems that hardly anyone (or even no one?) actually uses big endian
arm kernels.  But as long as they're ostensibly supported, we should
disable this code on them so that it's not accidentally used.

Note: for future-proofing, use !CPU_BIG_ENDIAN instead of
CPU_LITTLE_ENDIAN.  Both of these are arch-specific options that could
get removed in the future if big endian support gets dropped.

Fixes: d8f1308 ("crypto: arm/curve25519 - wire up NEON implementation")
Cc: stable@vger.kernel.org
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251104054906.716914-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: Eric Biggers

lib/crypto/Kconfig

@@ -64,7 +64,7 @@ config CRYPTO_LIB_CURVE25519
 config CRYPTO_LIB_CURVE25519_ARCH
 	bool
 	depends on CRYPTO_LIB_CURVE25519 && !UML && !KMSAN
-	default y if ARM && KERNEL_MODE_NEON
+	default y if ARM && KERNEL_MODE_NEON && !CPU_BIG_ENDIAN
 	default y if PPC64 && CPU_LITTLE_ENDIAN
 	default y if X86_64