Allow system server to access bpf fs

The permission to allow system_server to access sys/fs/bpf/ directory
is missing. Add it back so it can get the bpf maps from the bpf_fs.

Test: device boot and no more denial information of system_server try to
searcg in fs_bpf
      atest android.net.cts.TrafficStatsTest
Bug: 75285088

Change-Id: I1040cde6c038eccc4e91c69a10b20aa7a18b19f6