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>

arch/sparc/kernel/central.c

@@ -55,7 +55,7 @@ static int clock_board_calc_nslots(struct clock_board *p)
 			else
 				return 5;
 		}
-		/* Fallthrough */
+		fallthrough;
 	default:
 		return 4;
 	}