net: remove unnecessary Close contention.

Contention profile in BenchmarkTCPOneShot (Core 2 Quad):

Before
Total: 80.285 seconds
44.743  55.7%  55.7%   44.743  55.7% runtime.chanrecv1
31.995  39.9%  95.6%   31.995  39.9% sync.(*Mutex).Lock
 3.547   4.4% 100.0%    3.547   4.4% runtime.chansend1

After
Total: 48.341 seconds
45.810  94.8%  94.8%   45.810  94.8% runtime.chanrecv1
 2.530   5.2% 100.0%    2.530   5.2% runtime.chansend1
 0.001   0.0% 100.0%    0.001   0.0% sync.(*Mutex).Lock

R=golang-dev, dave, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6845119

src/pkg/net/fd_unix.go

@@ -375,8 +375,8 @@ func (fd *netFD) decref() {
 
 func (fd *netFD) Close() error {
 	fd.pollServer.Lock() // needed for both fd.incref(true) and pollserver.Evict
-	defer fd.pollServer.Unlock()
 	if err := fd.incref(true); err != nil {
+		fd.pollServer.Unlock()
 		return err
 	}
 	// Unblock any I/O.  Once it all unblocks and returns,
@@ -385,6 +385,7 @@ func (fd *netFD) Close() error {
 	// fairly quickly, since all the I/O is non-blocking, and any
 	// attempts to block in the pollserver will return errClosing.
 	fd.pollServer.Evict(fd)
+	fd.pollServer.Unlock()
 	fd.decref()
 	return nil
 }

src/pkg/net/sock_posix.go

@@ -61,7 +61,6 @@ func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr,
 		}
 		if err = fd.connect(ursa); err != nil {
 			closesocket(s)
-			fd.Close()
 			return nil, err
 		}
 		fd.isConnected = true