umb can use p2p_input as an if_input handler.

umb can use p2p_input as an if_input handler.

umb(4) is a hardware p2p driver, it just has ip coming in, so we can do
the same thing we do for the address family and input processing as
other p2p interfaces.

the short packet check that umb_input does is already done by the ip
stacks, so we're not losing anything.

tested by gerhard@ mbuhl@
ok gerhard@