I had a similar problem (but don't know if it's the same). Workaround for me was to disable use sendfile = yes in smb.conf (which should be default).
However, I didn't like this de-optimization and since it worked with kernel 3.15 for me, I did a kernel bisect between 3.15 and 3.16.
Turned out "[3ae8f4e0b98b640aadf410c21185ccb6b5b02351] net: mv643xx_eth: Implement software TSO" introduced the regression: TCP segmentation offloading emulation (not the networking chip does segmentation, but the driver emulates segmentation offloading in software AFAIK).
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$ ethtool -K eth0 tso off
which disables that TSO offloading emulation and works just fine.
As said previously: I don't know whether my problem is the same one you're facing, but it's worth a try