Replace fmt.Errorf w/o %-style to errors.New

Using fmt.Errorf for errors that do not have %-style formatting
directives is an overkill. Switch to errors.New.

Found by

	git grep fmt.Errorf | grep -v ^vendor | grep -v '%'

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process.go

@@ -1,7 +1,7 @@
 package libcontainer
 
 import (
-	"fmt"
+	"errors"
 	"io"
 	"math"
 	"os"
@@ -9,6 +9,8 @@ import (
 	"github.com/opencontainers/runc/libcontainer/configs"
 )
 
+var errInvalidProcess = errors.New("invalid process")
+
 type processOperations interface {
 	wait() (*os.ProcessState, error)
 	signal(sig os.Signal) error
@@ -84,7 +86,7 @@ type Process struct {
 // Wait releases any resources associated with the Process
 func (p Process) Wait() (*os.ProcessState, error) {
 	if p.ops == nil {
-		return nil, newGenericError(fmt.Errorf("invalid process"), NoProcessOps)
+		return nil, newGenericError(errInvalidProcess, NoProcessOps)
 	}
 	return p.ops.wait()
 }
@@ -94,7 +96,7 @@ func (p Process) Pid() (int, error) {
 	// math.MinInt32 is returned here, because it's invalid value
 	// for the kill() system call.
 	if p.ops == nil {
-		return math.MinInt32, newGenericError(fmt.Errorf("invalid process"), NoProcessOps)
+		return math.MinInt32, newGenericError(errInvalidProcess, NoProcessOps)
 	}
 	return p.ops.pid(), nil
 }
@@ -102,7 +104,7 @@ func (p Process) Pid() (int, error) {
 // Signal sends a signal to the Process.
 func (p Process) Signal(sig os.Signal) error {
 	if p.ops == nil {
-		return newGenericError(fmt.Errorf("invalid process"), NoProcessOps)
+		return newGenericError(errInvalidProcess, NoProcessOps)
 	}
 	return p.ops.signal(sig)
 }