target/riscv: Do not allow sfence.vma from user mode

target/riscv: Do not allow sfence.vma from user mode

The 'sfence.vma' instruction is privileged, and should only ever be allowed
when executing in supervisor mode or higher.

Signed-off-by: Jonathan Behrens <fintelia@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>