Label /sys/class/net as sysfs_net.

We already expect contents of /sys/class/net to be labeled as sysfs_net.
Also label the directory for consistensy since we usually label
/sys/class/foo directories as sysfs_foo.

Bug: 65643247
Test: netd_integration_test
Test: can browse internet without denials to sysfs_net
Change-Id: I9d28ab4baf71df99ae966276532f14684d1abca6

private/genfs_contexts

@@ -87,6 +87,7 @@ genfscon sysfs / u:object_r:sysfs:s0
 genfscon sysfs /devices/system/cpu u:object_r:sysfs_devices_system_cpu:s0
 genfscon sysfs /class/android_usb                 u:object_r:sysfs_android_usb:s0
 genfscon sysfs /class/leds                        u:object_r:sysfs_leds:s0
+genfscon sysfs /class/net                         u:object_r:sysfs_net:s0
 genfscon sysfs /class/rtc                         u:object_r:sysfs_rtc:s0
 genfscon sysfs /class/switch                      u:object_r:sysfs_switch:s0
 genfscon sysfs /devices/platform/nfc-power/nfc_power u:object_r:sysfs_nfc_power_writable:s0