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>

drivers/hwtracing/intel_th/sth.c

@@ -84,11 +84,11 @@ static ssize_t notrace sth_stm_packet(struct stm_data *stm_data,
 	/* Global packets (GERR, XSYNC, TRIG) are sent with register writes */
 	case STP_PACKET_GERR:
 		reg += 4;
-		/* fall through */
+		fallthrough;
 
 	case STP_PACKET_XSYNC:
 		reg += 8;
-		/* fall through */
+		fallthrough;
 
 	case STP_PACKET_TRIG:
 		if (flags & STP_PACKET_TIMESTAMPED)