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>

kernel/cpuset.c

@@ -1610,7 +1610,7 @@ out_unlock:
  * 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;