SUNRPC: Clean up rpc_verify_header()

- Recover some instruction count because I'm about to introduce a
  few xdr_inline_decode call sites
- Replace dprintk() call sites with trace points
- Reduce the hot path so it fits in fewer cachelines

I've also renamed it rpc_decode_header() to match everything else
in the RPC client.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>