io_uring: add end-of-bits marker and build time verify it

Not easy to tell if we're going over the size of bits we can shove
in req->flags, so add an end-of-bits marker and a BUILD_BUG_ON()
check for it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -511,6 +511,9 @@ enum {
 	REQ_F_OVERFLOW_BIT,
 	REQ_F_POLLED_BIT,
 	REQ_F_BUFFER_SELECTED_BIT,
+
+	/* not a real bit, just to check we're not overflowing the space */
+	__REQ_F_LAST_BIT,
 };
 
 enum {
@@ -7998,6 +8001,7 @@ static int __init io_uring_init(void)
 	BUILD_BUG_SQE_ELEM(44, __s32,  splice_fd_in);
 
 	BUILD_BUG_ON(ARRAY_SIZE(io_op_defs) != IORING_OP_LAST);
+	BUILD_BUG_ON(__REQ_F_LAST_BIT >= 8 * sizeof(int));
 	req_cachep = KMEM_CACHE(io_kiocb, SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 	return 0;
 };