libct: unify IOPriority setting

For some reason, io priority is set in different places between runc
start/run and runc exec:

 - for runc start/run, it is done in the middle of (*linuxStandardInit).Init,
   close to the place where we exec runc init.
 - for runc exec, it is done much earlier, in (*setnsProcess) start().

Let's move setIOPriority call for runc exec to (*linuxSetnsInit).Init,
so it is in the same logical place as for runc start/run.

Also, move the function itself to init_linux.go as it's part of init.

Should not have any visible effect, except part of runc init is run with
a different I/O priority.

While at it, rename setIOPriority to setupIOPriority, and make it accept
the whole *configs.Config, for uniformity with other similar functions.

Fixes: bfbd0305 ("Add I/O priority")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/init_linux.go

@@ -678,6 +678,28 @@ func setupScheduler(config *configs.Config) error {
 	return nil
 }
 
+func setupIOPriority(config *configs.Config) error {
+	const ioprioWhoPgrp = 1
+
+	ioprio := config.IOPriority
+	if ioprio == nil {
+		return nil
+	}
+	class, ok := configs.IOPrioClassMapping[ioprio.Class]
+	if !ok {
+		return fmt.Errorf("invalid io priority class: %s", ioprio.Class)
+	}
+
+	// Combine class and priority into a single value
+	// https://github.com/torvalds/linux/blob/v5.18/include/uapi/linux/ioprio.h#L5-L17
+	iop := (class << 13) | ioprio.Priority
+	_, _, errno := unix.RawSyscall(unix.SYS_IOPRIO_SET, ioprioWhoPgrp, 0, uintptr(iop))
+	if errno != 0 {
+		return fmt.Errorf("failed to set io priority: %w", errno)
+	}
+	return nil
+}
+
 func setupPersonality(config *configs.Config) error {
 	return system.SetLinuxPersonality(config.Personality.Domain)
 }