rxrpc: Improve up-front incoming packet checking

Do more up-front checking on incoming packets to weed out invalid ones and
also ones aimed at services that we don't support.

Whilst we're at it, replace the clearing of call and skew if we don't find
a connection with just initialising the variables to zero at the top of the
function.

Signed-off-by: David Howells <dhowells@redhat.com>