chelsio/l2t: Annotate struct l2t_data with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct l2t_data.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230929181149.3006432-1-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -76,7 +76,7 @@ struct l2t_data {
 	atomic_t nfree;		/* number of free entries */
 	rwlock_t lock;
 	struct rcu_head rcu_head;	/* to handle rcu cleanup */
-	struct l2t_entry l2tab[];
+	struct l2t_entry l2tab[] __counted_by(nentries);
 };
 
 typedef void (*arp_failure_handler_func)(struct t3cdev * dev,

drivers/net/ethernet/chelsio/cxgb4/l2t.c

@@ -59,7 +59,7 @@ struct l2t_data {
 	rwlock_t lock;
 	atomic_t nfree;             /* number of free entries */
 	struct l2t_entry *rover;    /* starting point for next allocation */
-	struct l2t_entry l2tab[];  /* MUST BE LAST */
+	struct l2t_entry l2tab[] __counted_by(l2t_size);  /* MUST BE LAST */
 };
 
 static inline unsigned int vlan_prio(const struct l2t_entry *e)