sched_ext: Guard the cid kfuncs against unallocated cid tables

The cid tables are allocated by the first enable's scx_cid_init(), which
runs after scx_alloc_and_add_sched() has published ops->priv. A TRACING or
SYSCALL program associated with the enabling struct_ops map gets a non-NULL
sched from scx_prog_sched() as soon as ops->priv is set, so a cid kfunc
called in that window dereferences the still-NULL table pointer. Only the
first enable since boot is exposed as the tables are never freed.

scx_bpf_this_cid() and scx_bpf_task_cid() already handle the window by
testing the table pointer. Do the same in scx_cid_to_cpu(),
scx_cpu_to_cid() and scx_bpf_cid_topo(), returning -EINVAL / all-(-1) topo
as before any scheduler is enabled. __scx_cid_to_cpu() and
__scx_cpu_to_cid() stay unchecked for callers with the tables guaranteed
allocated - ops invocations on a live scheduler and the enable path itself.

Fixes: e9b55af47edf ("sched_ext: Add topological CPU IDs (cids)")
Signed-off-by: Tejun Heo <tj@kernel.org>