cgroup: clean up obsolete comment for parse_cgroupfs_options()

1d5be6b2 ("cgroup: move module ref handling into
rebind_subsystems()") makes parse_cgroupfs_options() no longer takes
refcounts on subsystems.

And unified hierachy makes parse_cgroupfs_options not need to call
with cgroup_mutex held to protect the cgroup_subsys[].

So this patch removes BUG_ON() and the comment.  As the comment
doesn't contain useful information afterwards, the whole comment is
removed.
Signed-off-by: NJianyu Zhan <nasa4836@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -1221,12 +1221,6 @@ struct cgroup_sb_opts {
 	bool none;
 };
 
-/*
- * Convert a hierarchy specifier into a bitmask of subsystems and
- * flags. Call with cgroup_mutex held to protect the cgroup_subsys[]
- * array. This function takes refcounts on subsystems to be used, unless it
- * returns error, in which case no refcounts are taken.
- */
 static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 {
 	char *token, *o = data;
@@ -1235,8 +1229,6 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 	struct cgroup_subsys *ss;
 	int i;
 
-	BUG_ON(!mutex_is_locked(&cgroup_mutex));
-
 #ifdef CONFIG_CPUSETS
 	mask = ~(1UL << cpuset_cgrp_id);
 #endif