pkg/ns: evaluate syscall number at compile-time

Previously this code used a run-time map lookup keyed by
runtime.GOOS/GOARCH.  This version uses conditional compilation to make
this choice at compile time, giving immediate feedback for unsupported
platforms.

ns/ns.go

@@ -21,24 +21,9 @@ import (
 	"syscall"
 )
 
-var setNsMap = map[string]uintptr{
-	"386":   346,
-	"amd64": 308,
-	"arm":   375,
-}
-
 // SetNS sets the network namespace on a target file.
 func SetNS(f *os.File, flags uintptr) error {
-	if runtime.GOOS != "linux" {
-		return fmt.Errorf("unsupported OS: %s", runtime.GOOS)
-	}
-
-	trap, ok := setNsMap[runtime.GOARCH]
-	if !ok {
-		return fmt.Errorf("unsupported arch: %s", runtime.GOARCH)
-	}
-
-	_, _, err := syscall.RawSyscall(trap, f.Fd(), flags, 0)
+	_, _, err := syscall.RawSyscall(setNsNr, f.Fd(), flags, 0)
 	if err != 0 {
 		return err
 	}