io_uring: add build check for buf_index overflows

req->buf_index is u16 and so we rely on registered buffers indexes
fitting into it. Add a build check, so when the upper limit for the
number of buffers is lifted we get a compliation fail but not lurking
problems.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/787e8e1a17cea51ca6301426b1c4c4887b8bd676.1629920396.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -10493,6 +10493,10 @@ static int __init io_uring_init(void)
 		     sizeof(struct io_uring_rsrc_update));
 	BUILD_BUG_ON(sizeof(struct io_uring_rsrc_update) >
 		     sizeof(struct io_uring_rsrc_update2));
+
+	/* ->buf_index is u16 */
+	BUILD_BUG_ON(IORING_MAX_REG_BUFFERS >= (1u << 16));
+
 	/* should fit into one byte */
 	BUILD_BUG_ON(SQE_VALID_FLAGS >= (1 << 8));