Refactor the file_operations functions of fsocket

The fastsocket is created refer to the kernel socket. So we should
follow the kernel socket style.

1. Rename socket_file_ops variable to fsocket_file_ops;
2. Rename the file_operations functions refer to kernel socket
operation functions.
    The style of latter is sock_xxxx, so the former one should be the fsock_xxx
3. Declare the external function in one block;
4. The implementation of some fsocket operation functions just invoke
the kernel socket operation functions. Now use them directly. It could
decrease one function call. Although the old functions are defined
like inline, it will not be unfolded because it's used as callback.