SUNRPC: Rename xs_encode_tcp_fragment_header()

Clean up: Use a more generic name for xs_encode_tcp_fragment_header();
it's appropriate to use for all stream transport types.  We're about
to add new stream transport.

Also, move it to a place where it is more easily shared amongst the
various send_request methods.  And finally, replace the "htonl" macro
invocation with its modern equivalent.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>