fs/select: Annotate struct poll_list with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
entries to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://lore.kernel.org/r/20240808150023.72578-2-thorsten.blum@toblux.com
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/select.c

@@ -840,7 +840,7 @@ SYSCALL_DEFINE1(old_select, struct sel_arg_struct __user *, arg)
 struct poll_list {
 	struct poll_list *next;
 	unsigned int len;
-	struct pollfd entries[];
+	struct pollfd entries[] __counted_by(len);
 };
 
 #define POLLFD_PER_PAGE  ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd))