cgroup: drop const from @buffer of cftype->write_string()

cftype->write_string() just passes on the writeable buffer from kernfs
and there's no reason to add const restriction on the buffer.  The
only thing const achieves is unnecessarily complicating parsing of the
buffer.  Drop const from @buffer.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>                                           
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 60ea160..f575491 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2143,7 +2143,7 @@
 }
 
 static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
-				      struct cftype *cft, const char *buffer)
+				      struct cftype *cft, char *buffer)
 {
 	struct cgroup_root *root = css->cgroup->root;
 
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 2ea98b2..2bc4a22 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -442,7 +442,7 @@
 }
 
 static int freezer_write(struct cgroup_subsys_state *css, struct cftype *cft,
-			 const char *buffer)
+			 char *buffer)
 {
 	bool freeze;
 
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 8d53245..efbf9ba 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1610,7 +1610,7 @@
  * Common handling for a write to a "cpus" or "mems" file.
  */
 static int cpuset_write_resmask(struct cgroup_subsys_state *css,
-				struct cftype *cft, const char *buf)
+				struct cftype *cft, char *buf)
 {
 	struct cpuset *cs = css_cs(css);
 	struct cpuset *trialcs;