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>
staging: vme: mmap() support for vme_user

We also make sure that user won't be able to reconfigure the window that has
already been mmap'ed.

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