sched_ext: Reorganize enable/disable path for multi-scheduler support

sched_ext: Reorganize enable/disable path for multi-scheduler support

In preparation for multiple scheduler support, reorganize the enable and
disable paths to make scheduler instances explicit. Extract
scx_root_disable() from scx_disable_workfn(). Rename scx_enable_workfn()
to scx_root_enable_workfn(). Change scx_disable() to take @sch parameter
and only queue disable_work if scx_claim_exit() succeeds for consistency.
Move exit_kind validation into scx_claim_exit(). The sysrq handler now
prints a message when no scheduler is loaded.

These changes don't materially affect user-visible behavior.

v2: Keep scx_enable() name as-is and only rename the workfn to
    scx_root_enable_workfn(). Change scx_enable() return type to s32.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>