Fix metrics collection failure in network=host mode

When using hostinet (network=host), /proc/net/dev and /proc/net/snmp
may not be available inside the sandbox. Previously Statistics() returned
an error when these files were not opened, which propagated up and caused
the entire event/metrics collection to fail, breaking kubectl top pod.

Return zero-valued stats instead of an error when the proc files are
unavailable, as this is expected in hostinet mode. Real errors like
parse failures still propagate.

Fixes #12723

Author: zsxsoft

pkg/sentry/socket/hostinet/stack.go

@@ -320,12 +320,12 @@ func (s *Stack) Statistics(stat any, arg string) error {
 	switch stat.(type) {
 	case *inet.StatDev:
 		if s.netDevFile == nil {
-			return fmt.Errorf("/proc/net/dev is not opened for hostinet")
+			return nil
 		}
 		rawLine = getLine(s.netDevFile, arg, false /* with no header */)
 	case *inet.StatSNMPIP, *inet.StatSNMPICMP, *inet.StatSNMPICMPMSG, *inet.StatSNMPTCP, *inet.StatSNMPUDP, *inet.StatSNMPUDPLite:
 		if s.netSNMPFile == nil {
-			return fmt.Errorf("/proc/net/snmp is not opened for hostinet")
+			return nil
 		}
 		rawLine = getLine(s.netSNMPFile, arg, true)
 	default: