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/fs/adfs/dir_f.c b/fs/adfs/dir_f.c
index 30d526f..05e9634 100644
--- a/fs/adfs/dir_f.c
+++ b/fs/adfs/dir_f.c
@@ -18,11 +18,11 @@ static inline unsigned int adfs_readval(unsigned char *p, int len)
 
 	switch (len) {
 	case 4:		val |= p[3] << 24;
-			/* fall through */
+		fallthrough;
 	case 3:		val |= p[2] << 16;
-			/* fall through */
+		fallthrough;
 	case 2:		val |= p[1] << 8;
-			/* fall through */
+		fallthrough;
 	default:	val |= p[0];
 	}
 	return val;
@@ -32,11 +32,11 @@ static inline void adfs_writeval(unsigned char *p, int len, unsigned int val)
 {
 	switch (len) {
 	case 4:		p[3] = val >> 24;
-			/* fall through */
+		fallthrough;
 	case 3:		p[2] = val >> 16;
-			/* fall through */
+		fallthrough;
 	case 2:		p[1] = val >> 8;
-			/* fall through */
+		fallthrough;
 	default:	p[0] = val;
 	}
 }