cxgb3/l2t: Fix undefined behaviour

[ Upstream commit 76497732932f15e7323dc805e8ea8dc11bb587cf ]

The use of zero-sized array causes undefined behaviour when it is not
the last member in a structure. As it happens to be in this case.

Also, the current code makes use of a language extension to the C90
standard, but the preferred mechanism to declare variable-length
types such as this one is a flexible array member, 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. Which is beneficial
to cultivate a high-quality code.

Fixes: e48f129c ("[SCSI] cxgb3i: convert cdev->l2opt to use rcu to prevent NULL dereference")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>

drivers/net/ethernet/chelsio/cxgb3/l2t.h

@@ -75,8 +75,8 @@ struct l2t_data {
 	struct l2t_entry *rover;	/* starting point for next allocation */
 	atomic_t nfree;		/* number of free entries */
 	rwlock_t lock;
-	struct l2t_entry l2tab[0];
 	struct rcu_head rcu_head;	/* to handle rcu cleanup */
+	struct l2t_entry l2tab[];
 };
 
 typedef void (*arp_failure_handler_func)(struct t3cdev * dev,