arm64: svc: Ensure hardirq tracing is updated before return

We always run userspace with interrupts enabled, but with the recent
conversion of the syscall entry/exit code to C, we don't inform the
hardirq tracing code that interrupts are about to become enabled by
virtue of restoring the EL0 SPSR.

This patch ensures that trace_hardirqs_on() is called on the syscall
return path when we return to the assembly code with interrupts still
disabled.

Fixes: f37099b6 ("arm64: convert syscall trace logic to C")
Reported-by: NJulien Grall <julien.grall@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/syscall.c

@@ -91,8 +91,15 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
 		local_daif_mask();
 		flags = current_thread_info()->flags;
-		if (!has_syscall_work(flags))
+		if (!has_syscall_work(flags)) {
+			/*
+			 * We're off to userspace, where interrupts are
+			 * always enabled after we restore the flags from
+			 * the SPSR.
+			 */
+			trace_hardirqs_on();
 			return;
+		}
 		local_daif_restore(DAIF_PROCCTX);
 	}