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