tcg/riscv: Fix RISC-VH host build failure

Commit 269bd5d8 "cpu: Move the softmmu tlb to CPUNegativeOffsetState'
broke the RISC-V host build as there are two variables that are used but
not defined.

This patch renames the undefined variables mask_off and table_off to the
existing (but unused) mask_ofs and table_ofs variables.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <79729cc88ca509e08b5c4aa0aa8a52847af70c0f.1561039316.git.alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>