SUNRPC: Move TCP receive state variables into private data structure

Move the TCP receive state variables from the generic rpc_xprt structure to
a private structure maintained inside net/sunrpc/xprtsock.c.

Also rename a function/variable pair to refer to RPC fragment headers
instead of record markers, to be consistent with types defined in
sunrpc/*.h.

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