snapshot: prevent duplicate calls to mount returning different values

In addition to being wasteful, without this, concurrent calls to `Mount`
would have lost the previous `target` and so would never have been
removed.

Signed-off-by: Justin Chadwell <me@jedevc.com>

snapshot/localmounter_darwin.go

@@ -13,6 +13,10 @@ func (lm *localMounter) Mount() (string, error) {
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
 
+	if lm.target != "" {
+		return lm.target, nil
+	}
+
 	if lm.mounts == nil && lm.mountable != nil {
 		mounts, release, err := lm.mountable.Mount()
 		if err != nil {

snapshot/localmounter_freebsd.go

@@ -12,6 +12,10 @@ func (lm *localMounter) Mount() (string, error) {
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
 
+	if lm.target != "" {
+		return lm.target, nil
+	}
+
 	if lm.mounts == nil && lm.mountable != nil {
 		mounts, release, err := lm.mountable.Mount()
 		if err != nil {

snapshot/localmounter_linux.go

@@ -16,6 +16,10 @@ func (lm *localMounter) Mount() (string, error) {
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
 
+	if lm.target != "" {
+		return lm.target, nil
+	}
+
 	if lm.mounts == nil && lm.mountable != nil {
 		mounts, release, err := lm.mountable.Mount()
 		if err != nil {

snapshot/localmounter_windows.go

@@ -16,6 +16,10 @@ func (lm *localMounter) Mount() (string, error) {
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
 
+	if lm.target != "" {
+		return lm.target, nil
+	}
+
 	if lm.mounts == nil && lm.mountable != nil {
 		mounts, release, err := lm.mountable.Mount()
 		if err != nil {