It is valid to modify the remote segment that will be used with the
btl put/get operations as long as the resulting address range falls in
the originally prepared segment. Vader should have been calculating the
offset of the remote address in the registered region. This commit
fixes this issue.

Backported from open-mpi/ompi@cfbb9cba1610e8452b28950f0f3eb4de63671293
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.
Merge pull request #91 from hjelmn/vader_oshmem_fix

It is valid to modify the remote segment that will be used with the