base: deny region creation exceeding size of a ds

The rm_session implementation expects that offset + size must be
part of one dataspace. Unfortunately the parameters are not checked
properly during an rm::attach.

During an detach memory behind the actual region can be unmapped by such
bogus region entries.

Issues #591
noux: calculate rm::attach parameters correctly

Previous commit denies the creation of regions larger then the dataspace.
Noux does it by setting the default size to the dataspace size without
subtracting the offset.

Fixes #591