cgroup: add cgroup_parse_float()

cgroup already uses floating point for percent[ile] numbers and there
are several controllers which want to take them as input.  Add a
generic parse helper to handle inputs.

Update the interface convention documentation about the use of
percentage numbers.  While at it, also clarify the default time unit.

Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -131,6 +131,8 @@ void cgroup_free(struct task_struct *p);
 int cgroup_init_early(void);
 int cgroup_init(void);
 
+int cgroup_parse_float(const char *input, unsigned dec_shift, s64 *v);
+
 /*
  * Iteration helpers and macros.
  */