usbip: event handler as one thread

Dear all,

1. Overview

In current USB/IP implementation, event kernel threads are created for
each port. The functions of the threads are closing connection and
error handling so they don't have not so many events to handle. There's
no need to have thread for each port.

BEFORE) vhci side - VHCI_NPORTS(8) threads are created.
$ ps aux | grep usbip
root     10059  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10060  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10061  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10062  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10063  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10064  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10065  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]
root     10066  0.0  0.0      0     0 ?        S    17:06   0:00 [usbip_eh]

BEFORE) stub side - threads will be created every bind operation.
$ ps aux | grep usbip
root      8368  0.0  0.0      0     0 ?        S    17:56   0:00 [usbip_eh]
root      8399  0.0  0.0      0     0 ?        S    17:56   0:00 [usbip_eh]

This patch put event threads of stub and vhci driver as one workqueue.

AFTER) only one event threads in each vhci and stub side.
$ ps aux | grep usbip
root     10457  0.0  0.0      0     0 ?        S<   17:47   0:00 [usbip_event]

2. Modification to usbip_event.c

BEFORE) kernel threads are created in usbip_start_eh().

AFTER) one workqueue is created in new usbip_init_eh().

Event handler which was main loop of kernel thread is modified to
workqueue handler.

Events themselves are stored in struct usbip_device - same as before.
usbip_devices which have event are listed in event_list.

The handler picks an element from the list and wakeup usbip_device. The
wakeup method is same as before.

usbip_in_eh() substitutes statement which checks whether functions are
called from eh_ops or not. In this function, the worker context is used
for the checking. The context will be set in a variable in the
beginning of first event handling. usbip_in_eh() is used in event
handler so it works well.

3. Modifications to programs using usbip_event.c

Initialization and termination of workqueue are added to init and exit
routine of usbip_core respectively.

A. version info

v2)
# Merged 1/2 event handler itself and 2/2 user programs because of auto
build fail at 1/2 casued unmodified user programs in 1/2.

Signed-off-by: Nobuo Iwata <nobuo.iwata@fujixerox.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>