net: avoid array copy when shuffling SRV records

We don't need to shift array elements to shuffle them.
We just have to swap a selected element with 0th element.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/91750044

src/pkg/net/dnsclient.go

@@ -196,9 +196,7 @@ func (addrs byPriorityWeight) shuffleByWeight() {
 			s += int(addrs[i].Weight)
 			if s > n {
 				if i > 0 {
-					t := addrs[i]
-					copy(addrs[1:i+1], addrs[0:i])
-					addrs[0] = t
+					addrs[0], addrs[i] = addrs[i], addrs[0]
 				}
 				break
 			}