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.

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

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

net/sctp/sm_sideeffect.c

@@ -1516,7 +1516,7 @@ static int sctp_cmd_interpreter(enum sctp_event_type event_type,
 
 			if (timer_pending(timer))
 				break;
-			/* fall through */
+			fallthrough;
 
 		case SCTP_CMD_TIMER_START:
 			timer = &asoc->timers[cmd->obj.to];