powerpc/mm/radix: Fix build error when RADIX_MMU=n

The recent TLB flush rework broke the build when the Radix MMU is
disabled at build time, eg:

  (.text+0x264): undefined reference to `.radix__tlbiel_all'

We could add an empty version, but if we ever called it by accident
that would indicate a bad bug, so add a stub that just WARNs if we do.

Fixes: d4748276ae14 ("powerpc/64s: Improve local TLB flush for boot and MCE on POWER9")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/tlbflush-radix.h

@@ -11,7 +11,11 @@ static inline int mmu_get_ap(int psize)
 	return mmu_psize_defs[psize].ap;
 }
 
+#ifdef CONFIG_PPC_RADIX_MMU
 extern void radix__tlbiel_all(unsigned int action);
+#else
+static inline void radix__tlbiel_all(unsigned int action) { WARN_ON(1); };
+#endif
 
 extern void radix__flush_hugetlb_tlb_range(struct vm_area_struct *vma,
 					   unsigned long start, unsigned long end);