kfence, x86: only define helpers if !MODULE

x86's <asm/tlbflush.h> only declares non-module accessible functions
(such as flush_tlb_one_kernel) if !MODULE.

In preparation of including <asm/kfence.h> from the KFENCE test module,
only define the helpers if !MODULE to avoid breaking the build with
CONFIG_KFENCE_KUNIT_TEST=m.

Signed-off-by: Marco Elver <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Heiko Carstens <[e-mail]>

arch/x86/include/asm/kfence.h

···
 #ifndef _ASM_X86_KFENCE_H
 #define _ASM_X86_KFENCE_H
 
+#ifndef MODULE
+
 #include <linux/bug.h>
 #include <linux/kfence.h>
 
···
 	preempt_enable();
 	return true;
 }
+
+#endif /* !MODULE */
 
 #endif /* _ASM_X86_KFENCE_H */