dma-buf: make reservation_object_copy_fences rcu save

Stop requiring that the src reservation object is locked for this operation.

Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1504551766-5093-1-git-send-email-deathsimple@vodafone.de