seccomp: Stub for !CONFIG_SECCOMP

When using !CONFIG_SECCOMP with CONFIG_GENERIC_ENTRY, the
randconfig bots found the following snag:

   kernel/entry/common.c: In function 'syscall_trace_enter':
>> kernel/entry/common.c:52:23: error: implicit declaration
   of function '__secure_computing' [-Wimplicit-function-declaration]
      52 |                 ret = __secure_computing(NULL);
         |                       ^~~~~~~~~~~~~~~~~~

Since generic entry calls __secure_computing() unconditionally,
fix this by moving the stub out of the ifdef clause for
CONFIG_HAVE_ARCH_SECCOMP_FILTER so it's always available.

Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Linus Walleij <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: Linus Walleij

include/linux/seccomp.h

@@ -55,10 +55,10 @@ struct seccomp_data;
 
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 static inline int secure_computing(void) { return 0; }
-static inline int __secure_computing(const struct seccomp_data *sd) { return 0; }
 #else
 static inline void secure_computing_strict(int this_syscall) { return; }
 #endif
+static inline int __secure_computing(const struct seccomp_data *sd) { return 0; }
 
 static inline long prctl_get_seccomp(void)
 {