staging: usbip: use interrupt safe spinlock to avoid potential deadlock.

The function 'usbip_event_add()' may be called in interrupt context on
the stub side:
'stub_complete'->'stub_enqueue_ret_unlink'->'usbip_event_add'.
In this function it tries to get the lock 'ud->lock', so we should
disable irq when we get this lock in process context.

Signed-off-by: Harvey Yang <harvey.huawei.yang@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>