staging/usbip: convert to kthread

usbip has its own infrastructure for managing kernel
threads, similar to kthread. By changing it to use
the standard functions, we can simplify the code
and get rid of one of the last BKL users at the
same time.

Includes changes suggested by Max Vozeler.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Takahiro Hirofuchi <hirofuchi@users.sourceforge.net>
Cc: Max Vozeler <max@vozeler.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>