staging: usbip: Don't leak struct file.

usbip takes a reference on a struct file which is passed in via
sysfs.  Previously, this reference was never cleaned up, although
the socket it referred to was.

This patch drops the corresponding reference (found with the
socket's ->file backpointer) instead of just closing the socket.

Signed-off-by: Bernard Blackham <b-linuxgit@largestprime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>