block: optimise boundary blkdev_read_iter's checks

Combine pos and len checks and mark unlikely. Also, don't reexpand if
it's not truncated.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/fff34e613aeaae1ad12977dc4592cb1a1f5d3190.1634755800.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -503,16 +503,19 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	size_t shorted = 0;
 	ssize_t ret;
 
+	if (unlikely(pos + iov_iter_count(to) > size)) {
 		if (pos >= size)
 			return 0;
-
 		size -= pos;
 		if (iov_iter_count(to) > size) {
 			shorted = iov_iter_count(to) - size;
 			iov_iter_truncate(to, size);
 		}
+	}
 
 	ret = generic_file_read_iter(iocb, to);
+
+	if (unlikely(shorted))
 		iov_iter_reexpand(to, iov_iter_count(to) + shorted);
 	return ret;
 }