SUNRPC: Hoist init_decode out of svc_authenticate()

Now that each ->accept method has been converted to use xdr_stream,
the svcxdr_init_decode() calls can be hoisted back up into the
generic RPC server code.

The dprintk in svc_authenticate() is removed, since
trace_svc_authenticate() reports the same information.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>