sched_ext: Add scx_arena_to_kaddr() / scx_kaddr_to_arena()

sched_ext: Add scx_arena_to_kaddr() / scx_kaddr_to_arena()

Translating between a BPF-arena pointer and its kernel-side address is just
an add or subtract of the arena's kern_vm start. More such translations are
coming, so cache that start on scx_sched as @arena_kern_base at arena attach
and wrap both directions. Convert the existing open-coded subtraction in
scx_call_op_set_cpumask().

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