cputlb: Hoist tlb portions in tlb_flush_one_mmuidx_locked

cputlb: Hoist tlb portions in tlb_flush_one_mmuidx_locked

No functional change, but the smaller expressions make
the code easier to read.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>