usbip: vudc: Add SysFS infrastructure for VUDC

usbip: vudc: Add SysFS infrastructure for VUDC

Add sysfs attributes to allow controlling vudc from usbip tools.

dev_desc - device descriptor of current gadget. This is required to
	 be consisten with current usbip protocol and allow to list
	 exportable devices on given machine.

usbip_sockfd - allows to pass socket to kernel to start usbip transfer.

usbip_status - currnent status of device

This commit is a result of cooperation between Samsung R&D Institute
Poland and Open Operating Systems Student Society at University
of Warsaw (O2S3@UW) consisting of:

    Igor Kotrasinski <ikotrasinsk@gmail.com>
    Karol Kosik <karo9@interia.eu>
    Ewelina Kosmider <3w3lfin@gmail.com>
    Dawid Lazarczyk <lazarczyk.dawid@gmail.com>
    Piotr Szulc <ps347277@students.mimuw.edu.pl>

Tutor and project owner:
    Krzysztof Opasiak <k.opasiak@samsung.com>

Signed-off-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Karol Kosik <karo9@interia.eu>
[Various bug fixes, improvements and commit msg update]
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>