entry: Guard syscall_enter_audit() invocation with CONFIG_AUDITSYSCALL

entry: Guard syscall_enter_audit() invocation with CONFIG_AUDITSYSCALL

A bunch of older cross compilers notably RISCV64 and S390 fail to eliminate
the dead code when CONFIG_AUDITSYSCALL=n. The code in question is:

     if (unlikely(audit_context())
     	syscall_enter_audit(regs);

and in case of CONFIG_AUDITSYSCALL=n:

  static inline struct audit_context *audit_context(void)
  {
        return NULL;
  }

which should make the compiler eliminate the syscall_enter_audit()
call. But a RISV64 GCC12 cross compiler translates that into:

        if (unlikely(audit_context()))
    1c34:       00000097                auipc   ra,0x0
    1c38:       000080e7                jalr    ra # 1c34 <.L785>
    1c3c:       c511                    beqz    a0,1c48 <.L787>
                syscall_enter_audit(regs);
    1c3e:       8526                    mv      a0,s1
    1c40:       00000097                auipc   ra,0x0
    1c44:       000080e7                jalr    ra # 1c40 <.L785+0xc>

and then claims in the failing link:

   include/asm-generic/preempt.h:54:(.noinstr.text+0x1a20):
	undefined reference to 'syscall_enter_audit'

which is obviously hallucination.

Add an explicit IS_ENABLED(CONFIG_AUDITSYSCALL) check into the condition to
cure this compiler madness.

Fixes: 6f25517010dd ("entry: Rework syscall_audit_enter()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/87tso45bqq.ffs@fw13
Closes: https://lore.kernel.org/oe-kbuild-all/202609031938.ZvZZaRQy-lkp@intel.com/