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>

include/linux/mm.h

@@ -157,11 +157,14 @@ static inline void __mm_zero_struct_page(struct page *page)
 
 	switch (sizeof(struct page)) {
 	case 80:
-		_pp[9] = 0;	/* fallthrough */
+		_pp[9] = 0;
+		fallthrough;
 	case 72:
-		_pp[8] = 0;	/* fallthrough */
+		_pp[8] = 0;
+		fallthrough;
 	case 64:
-		_pp[7] = 0;	/* fallthrough */
+		_pp[7] = 0;
+		fallthrough;
 	case 56:
 		_pp[6] = 0;
 		_pp[5] = 0;