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>

arch/arc/kernel/disasm.c

@@ -339,7 +339,7 @@ void __kprobes disasm_instr(unsigned long addr, struct disasm_state *state,
 
 	case op_LDWX_S:	/* LDWX_S c, [b, u6] */
 		state->x = 1;
-		/* intentional fall-through */
+		fallthrough;
 
 	case op_LDW_S:	/* LDW_S c, [b, u6] */
 		state->zz = 2;