riscv: Flush the tlb when a page directory is freed

The riscv privileged specification mandates to flush the TLB whenever a
page directory is modified, so add that to tlb_flush().

Fixes: c5e9b2c2

 ("riscv: Improve tlb_flush()")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240128120405.25876-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>