btl/vader: when using knem allow write to segments registered with
prepare_src

This is a workaround to issues with the btl interface itself. There is
no way in 1.8 to tell the btl that a segment will be used both for
read and write. This workaround will allow segments prepared with
prepare_src to be used with both remote read and remote write.

There will be no master commit for this fix.