[riscv/regression] Missing tlb flush introduced in refactoring

Hello,

In qemu-system-riscv64, following a QEMU update, I get all sort of weird and not easily reproducible crashes in my risc-v guest.

I have bissected this issue to commit c7b951718815694284501ed01fec7acb8654db7b.
Some TLB flushes were removed in the following places:
target/riscv/cpu_helper.c: `csr_write_helper(env, s, CSR_MSTATUS);` -> `env->mstatus = s;` (twice)
target/riscv/op_helper.c: `csr_write_helper(env, s, CSR_MSTATUS);` -> `env->mstatus = s;` (twice)

Adding TLB flushes in all 4 places fixes the issues for me.