lib: checksum: hide unused expected_csum_ipv6_magic[]

When CONFIG_NET is disabled, an extra warning shows up for this
unused variable:

lib/checksum_kunit.c:218:18: error: 'expected_csum_ipv6_magic' defined but not used [-Werror=unused-const-variable=]

Replace the #ifdef with an IS_ENABLED() check that makes the compiler's
dead-code-elimination take care of the link failure.

Fixes: f24a7010 ("lib: checksum: Fix build with CONFIG_NET=n")
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/checksum_kunit.c

@@ -594,13 +594,15 @@ static void test_ip_fast_csum(struct kunit *test)
 
 static void test_csum_ipv6_magic(struct kunit *test)
 {
-#if defined(CONFIG_NET)
 	const struct in6_addr *saddr;
 	const struct in6_addr *daddr;
 	unsigned int len;
 	unsigned char proto;
 	__wsum csum;
 
+	if (!IS_ENABLED(CONFIG_NET))
+		return;
+
 	const int daddr_offset = sizeof(struct in6_addr);
 	const int len_offset = sizeof(struct in6_addr) + sizeof(struct in6_addr);
 	const int proto_offset = sizeof(struct in6_addr) + sizeof(struct in6_addr) +
@@ -618,7 +620,6 @@ static void test_csum_ipv6_magic(struct kunit *test)
 		CHECK_EQ(to_sum16(expected_csum_ipv6_magic[i]),
 			 csum_ipv6_magic(saddr, daddr, len, proto, csum));
 	}
-#endif /* !CONFIG_NET */
 }
 
 static struct kunit_case __refdata checksum_test_cases[] = {