rdma: ONCRPC RDMA protocol marshalling

This logic parses the ONCRDMA protocol headers that
precede the actual RPC header. It is placed in a separate
file to keep all protocol aware code in a single place.
Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>