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>