Rename the ioctl which sets volume properties from 'UBI_IOCSETPROP' to
'UBI_IOCSETVOLPROP' to reflect the fact that this ioctl is about volume
properties, not device properties. This is also consistent with the
other volume ioctl name - 'UBI_IOCVOLUP'.

The main motivation for the re-name, however, is that we are going
to introduce the per-UBI device "set properties" ioctl, so we need
good and logical naming.

At the same time, re-name the "set volume properties request" data
structure from 'struct ubi_set_prop_req' to
'struct ubi_set_vol_prop_req'.

And re-name 'UBI_PROP_DIRECT_WRITE' to 'UBI_VOL_PROP_DIRECT_WRITE'.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>