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/fs_context.c b/fs/fs_context.c
index 7d5c5dd..2834d1a 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -521,7 +521,7 @@ static int legacy_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	switch (param->type) {
 	case fs_value_is_string:
 		len = 1 + param->size;
-		/* Fall through */
+		fallthrough;
 	case fs_value_is_flag:
 		len += strlen(param->key);
 		break;