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>
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 7c6dd6f..7affaaf 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -795,7 +795,7 @@ static void scsi_io_completion_action(struct scsi_cmnd *cmd, int result)
 		}
 		if (!scsi_end_request(req, blk_stat, blk_rq_err_bytes(req)))
 			return;
-		/*FALLTHRU*/
+		fallthrough;
 	case ACTION_REPREP:
 		scsi_io_completion_reprep(cmd, q);
 		break;