riscv: optimize send_ipi_single

Don't go through send_ipi_mask, but just set the op bit and then pass
a simple generated hartid mask directly to sbi_send_ipi.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
[paul.walmsley@sifive.com: minor patch description fixes]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/kernel/smp.c

@@ -94,7 +94,13 @@ static void send_ipi_mask(const struct cpumask *mask, enum ipi_message_type op)
 
 static void send_ipi_single(int cpu, enum ipi_message_type op)
 {
-	send_ipi_mask(cpumask_of(cpu), op);
+	int hartid = cpuid_to_hartid_map(cpu);
+
+	smp_mb__before_atomic();
+	set_bit(op, &ipi_data[cpu].bits);
+	smp_mb__after_atomic();
+
+	sbi_send_ipi(cpumask_bits(cpumask_of(hartid)));
 }
 
 static inline void clear_ipi(void)