SUNRPC: Fix svcxdr_init_decode's end-of-buffer calculation

Ensure that stream-based argument decoding can't go past the actual
end of the receive buffer. xdr_init_decode's calculation of the
value of xdr->end over-estimates the end of the buffer because the
Linux kernel RPC server code does not remove the size of the RPC
header from rqstp->rq_arg before calling the upper layer's
dispatcher.

The server-side still uses the svc_getnl() macros to decode the
RPC call header. These macros reduce the length of the head iov
but do not update the total length of the message in the buffer
(buf->len).

A proper fix for this would be to replace the use of svc_getnl() and
friends in the RPC header decoder, but that would be a large and
invasive change that would be difficult to backport.

Fixes: 5191955d ("SUNRPC: Prepare for xdr_stream-style decoding on the server-side")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>