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/standard_init_linux.go

@@ -159,7 +159,7 @@ func (l *linuxStandardInit) Init() error {
 		return err
 	}
 
-	if err := setIOPriority(l.config.Config.IOPriority); err != nil {
+	if err := setupIOPriority(l.config.Config); err != nil {
 		return err
 	}