bpf: Refactor mark_{dynptr,iter}_read

There is possibility of sharing code between mark_dynptr_read and
mark_iter_read for updating liveness information of their stack slots.
Consolidate common logic into mark_stack_slot_obj_read function in
preparation for the next patch which needs the same logic for its own
stack slots.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204030400.208005-4-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>