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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/select.c

···
 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))