riscv: Add sfence.vma after early page table changes

This invalidates local TLB after modifying the page tables during early init as
it's too early to handle suprious faults as we otherwise do.

Fixes: f2c17aa ("RISC-V: Implement compile-time fixed mappings")
Reported-by: Syven Wang <syven.wang@sifive.com>
Signed-off-by: Syven Wang <syven.wang@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[Palmer: Cleaned up the commit text]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/mm/init.c

@@ -226,12 +226,11 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 
 	ptep = &fixmap_pte[pte_index(addr)];
 
-	if (pgprot_val(prot)) {
+	if (pgprot_val(prot))
 		set_pte(ptep, pfn_pte(phys >> PAGE_SHIFT, prot));
-	} else {
+	else
 		pte_clear(&init_mm, addr, ptep);
-		local_flush_tlb_page(addr);
-	}
+	local_flush_tlb_page(addr);
 }
 
 static pte_t *__init get_pte_virt(phys_addr_t pa)