bpf: reduce compiler warnings by adding fallthrough comments

Fixes the following warnings:

kernel/bpf/verifier.c: In function ‘may_access_direct_pkt_data’:
kernel/bpf/verifier.c:702:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (t == BPF_WRITE)
      ^
kernel/bpf/verifier.c:704:2: note: here
  case BPF_PROG_TYPE_SCHED_CLS:
  ^~~~
kernel/bpf/verifier.c: In function ‘reg_set_min_max_inv’:
kernel/bpf/verifier.c:2057:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
   true_reg->min_value = 0;
   ~~~~~~~~~~~~~~~~~~~~^~~
kernel/bpf/verifier.c:2058:2: note: here
  case BPF_JSGT:
  ^~~~
kernel/bpf/verifier.c:2068:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
   true_reg->min_value = 0;
   ~~~~~~~~~~~~~~~~~~~~^~~
kernel/bpf/verifier.c:2069:2: note: here
  case BPF_JSGE:
  ^~~~
kernel/bpf/verifier.c: In function ‘reg_set_min_max’:
kernel/bpf/verifier.c:2009:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
   false_reg->min_value = 0;
   ~~~~~~~~~~~~~~~~~~~~~^~~
kernel/bpf/verifier.c:2010:2: note: here
  case BPF_JSGT:
  ^~~~
kernel/bpf/verifier.c:2019:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
   false_reg->min_value = 0;
   ~~~~~~~~~~~~~~~~~~~~~^~~
kernel/bpf/verifier.c:2020:2: note: here
  case BPF_JSGE:
  ^~~~
Reported-by: NDavid Binderman <dcb314@hotmail.com>
Signed-off-by: NAlexander Alemayhu <alexander@alemayhu.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -701,6 +701,7 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 		/* dst_input() and dst_output() can't write for now */
 		if (t == BPF_WRITE)
 			return false;
+		/* fallthrough */
 	case BPF_PROG_TYPE_SCHED_CLS:
 	case BPF_PROG_TYPE_SCHED_ACT:
 	case BPF_PROG_TYPE_XDP:
@@ -2007,6 +2008,7 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
 	case BPF_JGT:
 		/* Unsigned comparison, the minimum value is 0. */
 		false_reg->min_value = 0;
+		/* fallthrough */
 	case BPF_JSGT:
 		/* If this is false then we know the maximum val is val,
 		 * otherwise we know the min val is val+1.
@@ -2017,6 +2019,7 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
 	case BPF_JGE:
 		/* Unsigned comparison, the minimum value is 0. */
 		false_reg->min_value = 0;
+		/* fallthrough */
 	case BPF_JSGE:
 		/* If this is false then we know the maximum value is val - 1,
 		 * otherwise we know the mimimum value is val.
@@ -2055,6 +2058,7 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
 	case BPF_JGT:
 		/* Unsigned comparison, the minimum value is 0. */
 		true_reg->min_value = 0;
+		/* fallthrough */
 	case BPF_JSGT:
 		/*
 		 * If this is false, then the val is <= the register, if it is
@@ -2066,6 +2070,7 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
 	case BPF_JGE:
 		/* Unsigned comparison, the minimum value is 0. */
 		true_reg->min_value = 0;
+		/* fallthrough */
 	case BPF_JSGE:
 		/* If this is false then constant < register, if it is true then
 		 * the register < constant.