accel/ivpu: Use effective buffer size for zero terminator

Use the effective written size instead of original size as index for zero
termination. If the input from user-space is to larger and the input is
truncated, the original size is out-of-bound.
Since there is an upfront size check here, the change is for consistency.

Signed-off-by: Markus Burri <markus.burri@mt.com>
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://lore.kernel.org/r/20250508130612.82270-3-markus.burri@mt.com

Author: Markus Burri

drivers/accel/ivpu/ivpu_debugfs.c

@@ -455,7 +455,7 @@ priority_bands_fops_write(struct file *file, const char __user *user_buf, size_t
 	if (ret < 0)
 		return ret;
 
-	buf[size] = '\0';
+	buf[ret] = '\0';
 	ret = sscanf(buf, "%u %u %u %u", &band, &grace_period, &process_grace_period,
 		     &process_quantum);
 	if (ret != 4)