ANDROID: dma-buf: Add mutex_unlock back which was being wrongly removed

dma_buf_attach() does not release his lock when returning in his normal
path. It will cause detaching thread blocked before llkd crashes kernel.

Bug: 194737348
Fixes: e618a51f (ANDROID: Delete the DMA-BUF attachment sysfs
statistics)

Change-Id: I41f20195888d35007f28c4e9f481add84958e728
Reported-by: Haidong.Yao <haidong.yao@unisoc.com>
Cc: Ping Wang <ping.wang@unisoc.com>
Cc: Ziwei Dai <ziwei.dai@unisoc.com>
Cc: Liangcai Fan <liangcai.fan@unisoc.com>
Signed-off-by: Orson Zhai <orson.zhai@unisoc.com>