s390/irq: inline do_softirq_own_stack()

Move do_softirq_own_stack() to proper header file so it can be
inlined; saving a few cycles.
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/softirq_stack.h

+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef __ASM_S390_SOFTIRQ_STACK_H
+#define __ASM_S390_SOFTIRQ_STACK_H
+
+#include <asm/lowcore.h>
+#include <asm/stacktrace.h>
+
+static inline void do_softirq_own_stack(void)
+{
+	CALL_ON_STACK(__do_softirq, S390_lowcore.async_stack, 0);
+}
+
+#endif /* __ASM_S390_SOFTIRQ_STACK_H */

arch/s390/kernel/irq.c

@@ -265,14 +265,6 @@ unsigned int arch_dynirq_lower_bound(unsigned int from)
 	return from < NR_IRQS_BASE ? NR_IRQS_BASE : from;
 }
 
-/*
- * Switch to the asynchronous interrupt stack for softirq execution.
- */
-void do_softirq_own_stack(void)
-{
-	CALL_ON_STACK(__do_softirq, S390_lowcore.async_stack, 0);
-}
-
 /*
  * ext_int_hash[index] is the list head for all external interrupts that hash
  * to this index.