Added socket type check

In some cases, where server is running with thousands of connections,
looping through all the file descriptors takes times. I have tested
on a server with 20k connections and it took around 1m30s to get all
socket information. After adding !strings.HasPrefix(lname, "socket")
to the loop the cost reduced significantly to  around 25 seconds.

Author: chicknsoup

netstat/netstat_linux.go

@@ -196,7 +196,7 @@ func (p *procFd) iterFdDir() {
 	for _, file := range fi {
 		fd := path.Join(fddir, file.Name())
 		lname, err := os.Readlink(fd)
-		if err != nil {
+		if err != nil || !strings.HasPrefix(lname, sockPrefix) {
 			continue
 		}