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/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index d0e2825..5299ef5 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3016,7 +3016,7 @@ static void string_registers_quirk(struct x86_emulate_ctxt *ctxt)
 	case 0xa4:	/* movsb */
 	case 0xa5:	/* movsd/w */
 		*reg_rmw(ctxt, VCPU_REGS_RSI) &= (u32)-1;
-		/* fall through */
+		fallthrough;
 	case 0xaa:	/* stosb */
 	case 0xab:	/* stosd/w */
 		*reg_rmw(ctxt, VCPU_REGS_RDI) &= (u32)-1;