svcrpc: close connection if client sends short packet

If the client sents a record too short to contain even the beginning of
the rpc header, then just close the connection.

The current code drops the record data and continues.  I don't see the
point.  It's a hopeless situation and simpler just to cut off the
connection completely.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>