util: fix race condition in WaitForFile

when using inotify, check for the file existence once the watcher is
configured.  It solves a race condition where the file is created the
watcher configuration and we'd not have any inotify event.

Closes: containers#2942

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/util.go

@@ -101,6 +101,12 @@ func WaitForFile(path string, chWait chan error, timeout time.Duration) (bool, e
 			inotifyEvents = watcher.Events
 		}
 		defer watcher.Close()
+
+		// Verify that the file wasn't created while we were setting up
+		// the watcher.
+		if _, err := os.Stat(path); err == nil {
+			return true, nil
+		}
 	}
 	if inotifyEvents == nil {
 		// If for any reason we fail to create the inotify