Remove useless fsocket_alloc, SOCKET_INODE, and INODE_SOCKET

1. The struct fsocket_alloc is same as the kernel struct socket_alloc
actually. So it is unnecessary to bring new one.
And the kernel also provide similar functions like SOCKET_INODE and
INODE_SOCKET. So they are useless too.
Actually you could find the INODE_SOCKET uses socket_alloc instead of
fsocket_alloc.

So remove the useless codes to clean.

2. Rename fastsocket_socket_cache to fsocket_socket_cache