The rdma_frag attached to the send request was not correctly released
upon request completion, leaking until MPI_Finalize. A quick solution
would have been to add RDMA_FRAG_RETURN at different locations on the
send request completion, but it would have unnecessarily made the
sendreq completion path more complex. Instead, I added the length to
the RDMA fragment so that it can be completed during the remote ack.
Be more explicit on the comment.
The rdma_frag can only be freed once when the peer forced a protocol
change (from RDMA GET to send/recv). Otherwise the fragment will be
returned once all data pertaining to it has been trasnferred.
Signed-off-by: George Bosilca <bosilca@icl.utk.edu>