bpf: verifier: avoid fall-through warnings

In preparation to enabling -Wimplicit-fallthrough, this patch silences
the following warning:

kernel/bpf/verifier.c: In function ‘check_return_code’:
kernel/bpf/verifier.c:6106:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (env->prog->expected_attach_type == BPF_CGROUP_UDP4_RECVMSG ||
      ^
kernel/bpf/verifier.c:6109:2: note: here
  case BPF_PROG_TYPE_CGROUP_SKB:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that is much clearer to explicitly add breaks in each case
statement (that actually contains some code), rather than letting
the code to fall through.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -6103,11 +6103,13 @@ static int check_return_code(struct bpf_verifier_env *env)
 		if (env->prog->expected_attach_type == BPF_CGROUP_UDP4_RECVMSG ||
 		    env->prog->expected_attach_type == BPF_CGROUP_UDP6_RECVMSG)
 			range = tnum_range(1, 1);
+		break;
 	case BPF_PROG_TYPE_CGROUP_SKB:
 		if (env->prog->expected_attach_type == BPF_CGROUP_INET_EGRESS) {
 			range = tnum_range(0, 3);
 			enforce_attach_type_range = tnum_range(2, 3);
 		}
+		break;
 	case BPF_PROG_TYPE_CGROUP_SOCK:
 	case BPF_PROG_TYPE_SOCK_OPS:
 	case BPF_PROG_TYPE_CGROUP_DEVICE: