KVM: s390: Add signal_exits counter

Add a signal_exits counter for s390, as exists on arm64, loongarch, mips,
powerpc, riscv and x86.

This is used by kvm_handle_signal_exit(), which we will use when we
later enable CONFIG_VIRT_XFER_TO_GUEST_WORK.

Signed-off-by: Andrew Donnellan <[e-mail]>
Reviewed-by: Janosch Frank <[e-mail]>
Signed-off-by: Janosch Frank <[e-mail]>

arch/s390/include/asm/kvm_host.h

···
 	u64 instruction_diagnose_500;
 	u64 instruction_diagnose_other;
 	u64 pfault_sync;
+	u64 signal_exits;
 };
 
 #define PGM_OPERATION			0x01

arch/s390/kvm/kvm-s390.c

···
 	STATS_DESC_COUNTER(VCPU, instruction_diagnose_308),
 	STATS_DESC_COUNTER(VCPU, instruction_diagnose_500),
 	STATS_DESC_COUNTER(VCPU, instruction_diagnose_other),
-	STATS_DESC_COUNTER(VCPU, pfault_sync)
+	STATS_DESC_COUNTER(VCPU, pfault_sync),
+	STATS_DESC_COUNTER(VCPU, signal_exits)
 };
 
 const struct kvm_stats_header kvm_vcpu_stats_header = {
···
 
 	if (signal_pending(current) && !rc) {
 		kvm_run->exit_reason = KVM_EXIT_INTR;
+		vcpu->stat.signal_exits++;
 		rc = -EINTR;
 	}