Add a generic mechanism for skipping over transport-specific headers
 when constructing an RPC request.  This removes another "xprt->stream"
 dependency.

 Test-plan:
 Write-intensive workload on a single mount point (try both UDP and
 TCP).
Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>