selftests/bpf: mark dynptr kfuncs __weak to make them optional on old kernels

Mark dynptr kfuncs as __weak to allow
verifier_global_subprogs/arg_ctx_{perf,kprobe,raw_tp} subtests to be
loadable on old kernels. Because bpf_dynptr_from_xdp() kfunc is used
from arg_tag_dynptr BPF program in progs/verifier_global_subprogs.c
*and* is not marked as __weak, loading any subtest from
verifier_global_subprogs fails on old kernels that don't have
bpf_dynptr_from_xdp() kfunc defined. Even if arg_tag_dynptr program
itself is not loaded, libbpf bails out on non-weak reference to
bpf_dynptr_from_xdp (that can't be resolved), which shared across all
programs in progs/verifier_global_subprogs.c.

So mark all dynptr-related kfuncs as __weak to unblock libbpf CI ([0]).
In the upcoming "kfunc in vmlinux.h" work we should make sure that
kfuncs are always declared __weak as well.

[0] https://github.com/libbpf/libbpf/actions/runs/7792673215/job/21251250831?pr=776#step:4:7961

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/bpf_kfuncs.h

···
  *  Error code
  */
 extern int bpf_dynptr_from_skb(struct __sk_buff *skb, __u64 flags,
-    struct bpf_dynptr *ptr__uninit) __ksym;
+    struct bpf_dynptr *ptr__uninit) __ksym __weak;
 
 /* Description
  *  Initializes an xdp-type dynptr
···
  *  Error code
  */
 extern int bpf_dynptr_from_xdp(struct xdp_md *xdp, __u64 flags,
-			       struct bpf_dynptr *ptr__uninit) __ksym;
+			       struct bpf_dynptr *ptr__uninit) __ksym __weak;
 
 /* Description
  *  Obtain a read-only pointer to the dynptr's data
···
  *  buffer if unable to obtain a direct pointer
  */
 extern void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, __u32 offset,
-			      void *buffer, __u32 buffer__szk) __ksym;
+			      void *buffer, __u32 buffer__szk) __ksym __weak;
 
 /* Description
  *  Obtain a read-write pointer to the dynptr's data
···
  *  buffer if unable to obtain a direct pointer
  */
 extern void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr *ptr, __u32 offset,
-			      void *buffer, __u32 buffer__szk) __ksym;
+			      void *buffer, __u32 buffer__szk) __ksym __weak;
 
-extern int bpf_dynptr_adjust(const struct bpf_dynptr *ptr, __u32 start, __u32 end) __ksym;
-extern bool bpf_dynptr_is_null(const struct bpf_dynptr *ptr) __ksym;
-extern bool bpf_dynptr_is_rdonly(const struct bpf_dynptr *ptr) __ksym;
-extern __u32 bpf_dynptr_size(const struct bpf_dynptr *ptr) __ksym;
-extern int bpf_dynptr_clone(const struct bpf_dynptr *ptr, struct bpf_dynptr *clone__init) __ksym;
+extern int bpf_dynptr_adjust(const struct bpf_dynptr *ptr, __u32 start, __u32 end) __ksym __weak;
+extern bool bpf_dynptr_is_null(const struct bpf_dynptr *ptr) __ksym __weak;
+extern bool bpf_dynptr_is_rdonly(const struct bpf_dynptr *ptr) __ksym __weak;
+extern __u32 bpf_dynptr_size(const struct bpf_dynptr *ptr) __ksym __weak;
+extern int bpf_dynptr_clone(const struct bpf_dynptr *ptr, struct bpf_dynptr *clone__init) __ksym __weak;
 
 /* Description
  *  Modify the address of a AF_UNIX sockaddr.