sockfd_lookup — Go from a file number to its socket slot
| struct socket * fsfuncsockfd_lookup ( | int fd, | 
| int * err ); | 
The file handle passed in is locked and the socket it is bound too is returned. If an error occurs the err pointer is overwritten with a negative errno code and NULL is returned. The function checks for both invalid handles and passing a handle which is not a socket.
On a success the socket object pointer is returned.