BACKPORT: treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

Only apply changes to bpf, not treewide.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Change-Id: I56f83316fa51615b5e0bc37762f321223f7c1a7f
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

kernel/bpf/cgroup.c

@@ -1855,7 +1855,7 @@ static bool cg_sockopt_is_valid_access(int off, int size,
 			return prog->expected_attach_type ==
 				BPF_CGROUP_GETSOCKOPT;
 		case offsetof(struct bpf_sockopt, optname):
-			/* fallthrough */
+			fallthrough;
 		case offsetof(struct bpf_sockopt, level):
 			if (size != size_default)
 				return false;

kernel/bpf/cpumap.c

@@ -266,7 +266,7 @@ static int cpu_map_bpf_prog_run_xdp(struct bpf_cpu_map_entry *rcpu,
 			break;
 		default:
 			bpf_warn_invalid_xdp_action(act);
-			/* fallthrough */
+			fallthrough;
 		case XDP_DROP:
 			xdp_return_frame(xdpf);
 			stats->drop++;

kernel/bpf/syscall.c

@@ -2056,7 +2056,7 @@ bpf_prog_load_check_attach(enum bpf_prog_type prog_type,
 	case BPF_PROG_TYPE_EXT:
 		if (expected_attach_type)
 			return -EINVAL;
-		/* fallthrough */
+		fallthrough;
 	default:
 		return 0;
 	}

kernel/bpf/verifier.c

@@ -12398,7 +12398,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 	default:
 		if (!prog_extension)
 			return -EINVAL;
-		/* fallthrough */
+		fallthrough;
 	case BPF_MODIFY_RETURN:
 	case BPF_LSM_MAC:
 	case BPF_TRACE_FENTRY: