libbpf: mark bpf_iter_num_{new,next,destroy} as __weak

Mark bpf_iter_num_{new,next,destroy}() kfuncs declared for
bpf_for()/bpf_repeat() macros as __weak to allow users to feature-detect
their presence and guard bpf_for()/bpf_repeat() loops accordingly for
backwards compatibility with old kernels.

Now that libbpf supports kfunc calls poisoning and better reporting of
unresolved (but called) kfuncs, declaring number iterator kfuncs in
bpf_helpers.h won't degrade user experience and won't cause unnecessary
kernel feature dependencies.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230418002148.3255690-7-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf_helpers.h

@@ -293,9 +293,9 @@ enum libbpf_tristate {
 
 struct bpf_iter_num;
 
-extern int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end) __ksym;
-extern int *bpf_iter_num_next(struct bpf_iter_num *it) __ksym;
-extern void bpf_iter_num_destroy(struct bpf_iter_num *it) __ksym;
+extern int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end) __weak __ksym;
+extern int *bpf_iter_num_next(struct bpf_iter_num *it) __weak __ksym;
+extern void bpf_iter_num_destroy(struct bpf_iter_num *it) __weak __ksym;
 
 #ifndef bpf_for_each
 /* bpf_for_each(iter_type, cur_elem, args...) provides generic construct for