IBMNAS: Remove call to set r/w permissions to all

During cinder volume create operation from a volume snapshot or
from an existing volume (volume clone operation), the ibmnas
driver sets 'rw' permissions to all, which is unnecessary and
also poses security concerns.
Fixing this issue, removing the calls to set rw permissions to
all during these operations and adding a call to set 'rw'
permissions only to the owner to make sure even if umask is set
at the filesystem level, which might deny 'rw' access to the owner
we explicitely set the required permissions on the volume file.

Change-Id: I0e5ba9262a298e088f7724ddeda3537afa4b023e
Closes-Bug: #1367238