SUNRPC: Rename skb_reader_t and friends

Clean-up:  hch suggested that the RPC client shouldn't pollute the name
space used by the generic skb manipulation routines in net/core/skbuff.c.

Rename a couple of types in xdr.h to adhere to this convention.

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