os: fix data race in epipecheck()

Fixes #3860.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6443051

src/pkg/os/file_unix.go

@@ -24,7 +24,7 @@ type file struct {
 	fd      int
 	name    string
 	dirinfo *dirInfo // nil unless directory being read
-	nepipe  int      // number of consecutive EPIPE in Write
+	nepipe  int32    // number of consecutive EPIPE in Write
 }
 
 // Fd returns the integer Unix file descriptor referencing the open file.