libbpf: refactor bpf_*_get_next_id() functions

In preparation for the introduction of a similar function for retrieving
the id of the next BTF object, consolidate the code from
bpf_prog_get_next_id() and bpf_map_get_next_id() in libbpf.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf.c

@@ -568,7 +568,7 @@ int bpf_prog_test_run_xattr(struct bpf_prog_test_run_attr *test_attr)
 	return ret;
 }
 
-int bpf_prog_get_next_id(__u32 start_id, __u32 *next_id)
+static int bpf_obj_get_next_id(__u32 start_id, __u32 *next_id, int cmd)
 {
 	union bpf_attr attr;
 	int err;
@@ -576,26 +576,21 @@ int bpf_prog_get_next_id(__u32 start_id, __u32 *next_id)
 	memset(&attr, 0, sizeof(attr));
 	attr.start_id = start_id;
 
-	err = sys_bpf(BPF_PROG_GET_NEXT_ID, &attr, sizeof(attr));
+	err = sys_bpf(cmd, &attr, sizeof(attr));
 	if (!err)
 		*next_id = attr.next_id;
 
 	return err;
 }
 
-int bpf_map_get_next_id(__u32 start_id, __u32 *next_id)
+int bpf_prog_get_next_id(__u32 start_id, __u32 *next_id)
 {
-	union bpf_attr attr;
-	int err;
-
-	memset(&attr, 0, sizeof(attr));
-	attr.start_id = start_id;
-
-	err = sys_bpf(BPF_MAP_GET_NEXT_ID, &attr, sizeof(attr));
-	if (!err)
-		*next_id = attr.next_id;
+	return bpf_obj_get_next_id(start_id, next_id, BPF_PROG_GET_NEXT_ID);
+}
 
-	return err;
+int bpf_map_get_next_id(__u32 start_id, __u32 *next_id)
+{
+	return bpf_obj_get_next_id(start_id, next_id, BPF_MAP_GET_NEXT_ID);
 }
 
 int bpf_prog_get_fd_by_id(__u32 id)