ext4: use flexible-array member for xattr structs

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/20200309180813.GA3347@embeddedorSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/xattr.h

@@ -48,7 +48,7 @@ struct ext4_xattr_entry {
 	__le32	e_value_inum;	/* inode in which the value is stored */
 	__le32	e_value_size;	/* size of attribute value */
 	__le32	e_hash;		/* hash value of name and value */
-	char	e_name[0];	/* attribute name */
+	char	e_name[];	/* attribute name */
 };
 
 #define EXT4_XATTR_PAD_BITS		2
@@ -118,7 +118,7 @@ struct ext4_xattr_ibody_find {
 
 struct ext4_xattr_inode_array {
 	unsigned int count;		/* # of used items in the array */
-	struct inode *inodes[0];
+	struct inode *inodes[];
 };
 
 extern const struct xattr_handler ext4_xattr_user_handler;