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/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index 95f0974..548de75 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -2737,7 +2737,7 @@ static bool allocate_vpe_l2_table(int cpu, u32 id)
 	switch (gpsz) {
 	default:
 		WARN_ON(1);
-		/* fall through */
+		fallthrough;
 	case GIC_PAGE_SIZE_4K:
 		psz = SZ_4K;
 		break;
@@ -2832,7 +2832,7 @@ static int allocate_vpe_l1_table(void)
 	switch (gpsz) {
 	default:
 		gpsz = GIC_PAGE_SIZE_4K;
-		/* fall through */
+		fallthrough;
 	case GIC_PAGE_SIZE_4K:
 		psz = SZ_4K;
 		break;