UBI: allow direct user-space I/O

Introduce a new ioctl UBI_IOCSETPROP to set properties
on a volume. Also add the first property:
UBI_PROP_DIRECT_WRITE, this property is used to set the
ability to use direct writes in userspace

Signed-off-by: Sidney Amani <seed@uffs.org>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --cc drivers/mtd/ubi/Kconfig.debug
Simple merge

diff --cc drivers/mtd/ubi/cdev.c
Simple merge

diff --cc drivers/mtd/ubi/ubi.h
Simple merge

diff --cc include/mtd/ubi-user.h
Simple merge