[NET]: use fget_light() in net/socket.c

Here's an updated copy of the patch to use fget_light in net/socket.c.
Rerunning the tests show a drop of ~80Mbit/s on average, which looks
bad until you see the drop in cpu usage from ~89% to ~82%.  That will
get fixed in another patch...

Before: max 8113.70, min 8026.32, avg 8072.34
 87380  16384  16384    10.01      8045.55   87.11    87.11    1.774   1.774
 87380  16384  16384    10.01      8065.14   90.86    90.86    1.846   1.846
 87380  16384  16384    10.00      8077.76   89.85    89.85    1.822   1.822
 87380  16384  16384    10.00      8026.32   89.80    89.80    1.833   1.833
 87380  16384  16384    10.01      8108.59   89.81    89.81    1.815   1.815
 87380  16384  16384    10.01      8034.53   89.01    89.01    1.815   1.815
 87380  16384  16384    10.00      8113.70   90.45    90.45    1.827   1.827
 87380  16384  16384    10.00      8111.37   89.90    89.90    1.816   1.816
 87380  16384  16384    10.01      8077.75   87.96    87.96    1.784   1.784
 87380  16384  16384    10.00      8062.70   90.25    90.25    1.834   1.834

After: max 8035.81, min 7963.69, avg 7998.14
 87380  16384  16384    10.01      8000.93   82.11    82.11    1.682   1.682
 87380  16384  16384    10.01      8016.17   83.67    83.67    1.710   1.710
 87380  16384  16384    10.01      7963.69   83.47    83.47    1.717   1.717
 87380  16384  16384    10.01      8014.35   81.71    81.71    1.671   1.671
 87380  16384  16384    10.00      7967.68   83.41    83.41    1.715   1.715
 87380  16384  16384    10.00      7995.22   81.00    81.00    1.660   1.660
 87380  16384  16384    10.00      8002.61   83.90    83.90    1.718   1.718
 87380  16384  16384    10.00      8035.81   81.71    81.71    1.666   1.666
 87380  16384  16384    10.01      8005.36   82.56    82.56    1.690   1.690
 87380  16384  16384    10.00      7979.61   82.50    82.50    1.694   1.694
Signed-off-by: Benjamin LaHaise <bcrl@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>