net/net: Add SocketReadState for reuse codes

This function is from net/socket.c, move it to net.c and net.h.
Add SocketReadState to make others reuse net_fill_rstate().
suggestion from jason.

v4:
 - move 'rs->finalize = finalize' to rs_init()

v3:
 - remove SocketReadState init callback
 - put finalize callback to net_fill_rstate()

v2:
 - rename ReadState to SocketReadState
 - add SocketReadState init and finalize callback

v1:
 - init patch

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>