drm/amdkfd: Fix error handling in kfd_criu_restore_events

mutex_unlock before the exit label because all the error code paths that
jump there didn't take that lock. This fixes unbalanced locking errors
in case of restore errors.

Fixes: 40e8a766 ("drm/amdkfd: CRIU checkpoint and restore events")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>