bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena()

struct bpf_arena is opaque to callers outside arena.c. Add two helpers
for struct_ops subsystems that need to reach into an arena:

  bpf_arena_map_kern_vm_start(struct bpf_map *map)
    returns @map's kern_vm_start. A sched_ext follow-up needs this
    to translate kern_va <-> uaddr.

  bpf_prog_arena(struct bpf_prog *prog)
    returns the bpf_map of the arena referenced by @prog (NULL if
    @prog references no arena). The verifier enforces at most one
    arena per program. Used by struct_ops callers that auto-discover
    an arena from a member prog and need to take a map reference.

Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260522172219.1423324-6-tj@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>