ext4: use flexible-array members in struct dx_node and struct dx_root

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200213160648.GA7054@embeddedorSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/namei.c

@@ -233,13 +233,13 @@ struct dx_root
 		u8 unused_flags;
 	}
 	info;
-	struct dx_entry	entries[0];
+	struct dx_entry	entries[];
 };
 
 struct dx_node
 {
 	struct fake_dirent fake;
-	struct dx_entry	entries[0];
+	struct dx_entry	entries[];
 };