RISC-V: Fix handling of empty cpu masks

The cpumask rework slightly changed the behavior of the code.  Fix this
by treating an empty cpumask as meaning all online CPUs.

Extracted from a patch by Atish Patra <atishp@rivosinc.com>.

Reported-by: Jessica Clarke <jrtc27@jrtc27.com>
Fixes: 26fb751 ("RISC-V: Do not use cpumask data structure for hartid bitmap")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/sbi.c

@@ -161,7 +161,7 @@ static int __sbi_send_ipi_v01(const struct cpumask *cpu_mask)
 {
 	unsigned long hart_mask;
 
-	if (!cpu_mask)
+	if (!cpu_mask || cpumask_empty(cpu_mask))
 		cpu_mask = cpu_online_mask;
 	hart_mask = __sbi_v01_cpumask_to_hartmask(cpu_mask);
 
@@ -177,7 +177,7 @@ static int __sbi_rfence_v01(int fid, const struct cpumask *cpu_mask,
 	int result = 0;
 	unsigned long hart_mask;
 
-	if (!cpu_mask)
+	if (!cpu_mask || cpumask_empty(cpu_mask))
 		cpu_mask = cpu_online_mask;
 	hart_mask = __sbi_v01_cpumask_to_hartmask(cpu_mask);
 
@@ -254,7 +254,7 @@ static int __sbi_send_ipi_v02(const struct cpumask *cpu_mask)
 	struct sbiret ret = {0};
 	int result;
 
-	if (!cpu_mask)
+	if (!cpu_mask || cpumask_empty(cpu_mask))
 		cpu_mask = cpu_online_mask;
 
 	for_each_cpu(cpuid, cpu_mask) {
@@ -348,7 +348,7 @@ static int __sbi_rfence_v02(int fid, const struct cpumask *cpu_mask,
 	unsigned long hartid, cpuid, hmask = 0, hbase = 0;
 	int result;
 
-	if (!cpu_mask)
+	if (!cpu_mask || cpumask_empty(cpu_mask))
 		cpu_mask = cpu_online_mask;
 
 	for_each_cpu(cpuid, cpu_mask) {