gpu: host1x: Avoid stack over-read in debug output helpers

In the Linux kernel, the following vulnerability has been resolved: gpu: host1x: Avoid stack over-read in debug output helpers host1x_debug_output() and host1x_debug_cont() used vsnprintf(), which returns the length the formatted string would have reached with an unbounded buffer. That return value was passed straight to o->fn as the number of bytes to emit. This could cause a read past end of the output buffer if a call to host1x_debug_* produced a string longer than 256 bytes. This only affected the debugfs files as the printk debug sink ignores the number of bytes. In practice, this is very unlikely to occur. Fix by switching to vscnprintf(), which returns the number of bytes actually written.