sched: Make cpu_shares_read_u64() use tg_weight()

Move tg_weight() upward and make cpu_shares_read_u64() use it too. This
makes the weight retrieval shared between cgroup v1 and v2 paths and will be
used to implement cgroup support for sched_ext.

No functional changes.

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