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/ide/ide-floppy.c

@@ -428,7 +428,7 @@ static int ide_floppy_get_capacity(ide_drive_t *drive)
 				 * (maintains previous driver behaviour)
 				 */
 				break;
-			/* fall through */
+			fallthrough;
 		case CAPACITY_CURRENT:
 			/* Normal Zip/LS-120 disks */
 			if (memcmp(cap_desc, &floppy->cap_desc, 8))