riscv: Ensure page table writes are flushed when initializing KASAN vmalloc

Make sure that writes to kernel page table during KASAN vmalloc
initialization are made visible by adding a sfence.vma.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Fixes: e178d67 ("riscv/kasan: add KASAN_VMALLOC support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

Author: AlexGhiti

arch/riscv/mm/kasan_init.c

@@ -187,6 +187,8 @@ void __init kasan_shallow_populate(void *start, void *end)
 		}
 		vaddr += PAGE_SIZE;
 	}
+
+	local_flush_tlb_all();
 }
 
 void __init kasan_init(void)