dmaengine: virt-dma: Add missing locking around list operations

All list operations are protected by &vc->lock. As vchan_vdesc_fini()
is called unlocked add the missing locking around the list operations.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191216105328.15198-6-s.hauer@pengutronix.de


Signed-off-by: Vinod Koul <vkoul@kernel.org>