libbpf: Add support for extended BPF syscall

To support the extended BPF syscall introduced in the previous commit,
introduce the following internal APIs:

* 'sys_bpf_ext()'
* 'sys_bpf_ext_fd()'
  They wrap the raw 'syscall()' interface to support passing extended
  attributes.
* 'probe_sys_bpf_ext()'
  Check whether current kernel supports the BPF syscall common attributes.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260512153157.28382-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/features.c

@@ -615,6 +615,11 @@ static int probe_kern_btf_layout(int token_fd)
 						 (char *)layout, token_fd));
 }
 
+static int probe_bpf_syscall_common_attrs(int token_fd)
+{
+	return probe_sys_bpf_ext();
+}
+
 typedef int (*feature_probe_fn)(int /* token_fd */);
 
 static struct kern_feature_cache feature_cache;
@@ -699,6 +704,9 @@ static struct kern_feature_desc {
 	[FEAT_BTF_LAYOUT] = {
 		"kernel supports BTF layout", probe_kern_btf_layout,
 	},
+	[FEAT_BPF_SYSCALL_COMMON_ATTRS] = {
+		"BPF syscall common attributes support", probe_bpf_syscall_common_attrs,
+	},
 };
 
 bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)