Support customized attributes for CpuController and MemController

Customized attributes are useful for customized kernels.

Usage:
	let resource = &mut cgroups::Resources::default();
	resource.cpu.attrs.insert("cpu.cfs_init_buffer_us", "10".to_string());
	// apply here

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/cpu.rs

@@ -18,8 +18,8 @@ use crate::error::*;
 use crate::{parse_max_value, read_i64_from};
 
 use crate::{
-    ControllIdentifier, ControllerInternal, Controllers, CpuResources, MaxValue, Resources,
-    Subsystem,
+    ControllIdentifier, ControllerInternal, Controllers, CpuResources, CustomizedAttribute,
+    MaxValue, Resources, Subsystem,
 };
 
 /// A controller that allows controlling the `cpu` subsystem of a Cgroup.
@@ -91,6 +91,10 @@ impl ControllerInternal for CpuController {
                 return Err(Error::new(ErrorKind::Other));
             }
 
+            res.attrs.iter().for_each(|(k, v)| {
+                let _ = self.set(k, v);
+            })
+
             // TODO: rt properties (CONFIG_RT_GROUP_SCHED) are not yet supported
         }
 
@@ -306,6 +310,8 @@ impl CpuController {
     }
 }
 
+impl CustomizedAttribute for CpuController {}
+
 fn parse_cfs_quota_and_period(mut file: File) -> Result<CFSQuotaAndPeriod> {
     let mut content = String::new();
     file.read_to_string(&mut content)