bpf: Verifier, refine 32bit bound in do_refine_retval_range

Further refine return values range in do_refine_retval_range by noting
these are int return types (We will assume here that int is a 32-bit type).

Two reasons to pull this out of original patch. First it makes the original
fix impossible to backport. And second I've not seen this as being problematic
in practice unlike the other case.

Fixes: 849fa506 ("bpf/verifier: refine retval R0 state for bpf_get_stack helper")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158560421952.10843.12496354931526965046.stgit@john-Precision-5820-Tower