drm/v3d: Protect per-fd reset counter against fd release

drm/v3d: Protect per-fd reset counter against fd release

The per-fd reset counter tracks GPU resets caused by jobs submitted
through a specific file descriptor. However, there's a race condition
where the file descriptor can be closed while jobs are still running,
leading to potential access to freed memory when updating the reset
counter.

Ensure that the per-fd reset counter is only updated when the file
descriptor is still valid, preventing use-after-free scenarios during
GPU reset handling.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://lore.kernel.org/r/20250826-v3d-queue-lock-v3-6-979efc43e490@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>