staging: vme: use image mutex for ioctl()

This implements more granual locking in vme_user_ioctl() by using separate
locks for each sysfs device.

This also provides a synchronization between vme_user_read, vme_user_write and
vme_user_ioctl.

Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Cc: Igor Alekseev <igor.alekseev@itep.ru>