reiserfs: Replace one-element array with flexible-array member

One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element array with flexible-array
member in direntry_uarea structure, and refactor the rest of the code,
accordingly.

Worth mentioning is that before these changes, the original implementation
was returning two-too many bytes in function direntry_create_vi():

fs/reiserfs/item_ops.c:464:     int size = sizeof(struct direntry_uarea);
...
fs/reiserfs/item_ops.c-490-     size += (dir_u->entry_count * sizeof(short));
...
fs/reiserfs/item_ops.c-517-     return size;

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/290Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

fs/reiserfs/fix_node.c

@@ -2252,8 +2252,9 @@ static int get_virtual_node_size(struct super_block *sb, struct buffer_head *bh)
 
 	return sizeof(struct virtual_node) +
 	    max(max_num_of_items * sizeof(struct virtual_item),
-		sizeof(struct virtual_item) + sizeof(struct direntry_uarea) +
-		(max_num_of_entries - 1) * sizeof(__u16));
+		sizeof(struct virtual_item) +
+		struct_size_t(struct direntry_uarea, entry_sizes,
+			      max_num_of_entries));
 }
 
 /*

fs/reiserfs/reiserfs.h

@@ -2373,7 +2373,7 @@ struct virtual_node {
 struct direntry_uarea {
 	int flags;
 	__u16 entry_count;
-	__u16 entry_sizes[1];
+	__u16 entry_sizes[];
 } __attribute__ ((__packed__));
 
 /***************************************************************************