Sanity check RDMA remote data

Perform some basic (but probably not complete) sanity checking on
requests from the RDMA source.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>