ext4: make variable "count" signed

ext4: make variable “count” signed

This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven.
This ID is fixed in Linux Kernel version v4.9.320 by commit 984ceb2fc8f2b8cd7d38f9702ba5586c30804153, it was introduced in version v4.9.318 by commit 93bbf0498ba20eadcd7132bd3cfdaff54eb72751. For more details please see the references link.