sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()

scx_bpf_cpuperf_set() can be used to set a performance target level on
any CPU. However, it doesn't correctly acquire the corresponding rq
lock, which may lead to unsafe behavior and trigger the following
warning, due to the lockdep_assert_rq_held() check:

[   51.713737] WARNING: CPU: 3 PID: 3899 at kernel/sched/sched.h:1512 scx_bpf_cpuperf_set+0x1a0/0x1e0
...
[   51.713836] Call trace:
[   51.713837]  scx_bpf_cpuperf_set+0x1a0/0x1e0 (P)
[   51.713839]  bpf_prog_62d35beb9301601f_bpfland_init+0x168/0x440
[   51.713841]  bpf__sched_ext_ops_init+0x54/0x8c
[   51.713843]  scx_ops_enable.constprop.0+0x2c0/0x10f0
[   51.713845]  bpf_scx_reg+0x18/0x30
[   51.713847]  bpf_struct_ops_link_create+0x154/0x1b0
[   51.713849]  __sys_bpf+0x1934/0x22a0

Fix by properly acquiring the rq lock when possible or raising an error
if we try to operate on a CPU that is not the one currently locked.

Fixes: d86adb4f ("sched_ext: Add cpuperf support")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>