[NET]: Make use of ->private_data in sockfd_lookup

Please consider the patch below which makes use of file->private_data to
store the pointer to the socket, which avoids touching several unused
cachelines in the dentry and inode in sockfd_lookup.

Signed-off-by: Benjamin LaHaise <bcrl@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>