SUNRPC: Make rpcb_decode_getaddr more picky about universal addresses

Add better sanity checking of server replies to the GETVERSADDR reply
decoder.  Change the error return code: EIO is what other XDR decoding
routines return if there is a failure while decoding.

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