net: make deadline helpers join to netFD as its methods

Just for readability reasons; to prevent overlooking deadline stuff
across over platforms.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/8656044
diff --git a/src/pkg/net/fd_plan9.go b/src/pkg/net/fd_plan9.go
index e9527a3..84987c3 100644
--- a/src/pkg/net/fd_plan9.go
+++ b/src/pkg/net/fd_plan9.go
@@ -108,15 +108,15 @@
 	return os.NewFile(uintptr(dfd), s), nil
 }
 
-func setDeadline(fd *netFD, t time.Time) error {
+func (fd *netFD) setDeadline(t time.Time) error {
 	return syscall.EPLAN9
 }
 
-func setReadDeadline(fd *netFD, t time.Time) error {
+func (fd *netFD) setReadDeadline(t time.Time) error {
 	return syscall.EPLAN9
 }
 
-func setWriteDeadline(fd *netFD, t time.Time) error {
+func (fd *netFD) setWriteDeadline(t time.Time) error {
 	return syscall.EPLAN9
 }