enable gocritic linter

The linter ensures a common code style.
- use switch/case instead of else if
- use if instead of switch/case for single case statement
- add space between comment and text
- detect the use of defer with os.Exit()
- use short form var += "..." instead of var = var + "..."
- detect problems with append()
```
newSlice := append(orgSlice, val)
```
  This could lead to nasty bugs because the orgSlice will be changed in
  place if it has enough capacity too hold the new elements. Thus we
  newSlice might not be a copy.

Of course most of the changes are just cosmetic and do not cause any
logic errors but I think it is a good idea to enforce a common style.
This should help maintainability.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

pkg/specgen/generate/namespaces.go

@@ -202,10 +202,8 @@ func namespaceOptions(s *specgen.SpecGenerator, rt *libpod.Runtime, pod *libpod.
 	if s.IDMappings != nil {
 		if pod == nil {
 			toReturn = append(toReturn, libpod.WithIDMappings(*s.IDMappings))
-		} else {
-			if pod.HasInfraContainer() && (len(s.IDMappings.UIDMap) > 0 || len(s.IDMappings.GIDMap) > 0) {
-				return nil, errors.Wrapf(define.ErrInvalidArg, "cannot specify a new uid/gid map when entering a pod with an infra container")
-			}
+		} else if pod.HasInfraContainer() && (len(s.IDMappings.UIDMap) > 0 || len(s.IDMappings.GIDMap) > 0) {
+			return nil, errors.Wrapf(define.ErrInvalidArg, "cannot specify a new uid/gid map when entering a pod with an infra container")
 		}
 	}
 	if s.User != "" {
@@ -482,7 +480,7 @@ func GetNamespaceOptions(ns []string, netnsIsHost bool) ([]libpod.PodCreateOptio
 	var options []libpod.PodCreateOption
 	var erroredOptions []libpod.PodCreateOption
 	if ns == nil {
-		//set the default namespaces
+		// set the default namespaces
 		ns = strings.Split(specgen.DefaultKernelNamespaces, ",")
 	}
 	for _, toShare := range ns {