riscv: Optimize switch_mm by passing "cpu" to flush_icache_deferred()

Directly passing the cpu to flush_icache_deferred() rather than calling
smp_processor_id() again.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
[Palmer: drop the QEMU performance numbers, and update the comment]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>