staging: vme: use image mutex for ioctl()

This implements more granular locking in vme_user_ioctl() by using separate
locks for each devfs 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: Martyn Welch <martyn.welch@ge.com>
Cc: Igor Alekseev <igor.alekseev@itep.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>