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
(cherry picked from aosp commit f83bbd17)

private/system_server.te

@@ -756,6 +756,7 @@ with_asan(`
 
 # allow system_server to read the eBPF maps that stores the traffic stats information amd clean up
 # the map after snapshot is recorded
+allow system_server fs_bpf:dir search;
 allow system_server fs_bpf:file read;
 allow system_server netd:bpf map_read;