udp_var: correct intoudpcb macro unintended identifier dependency

Change 483fe9651 embedded struct inpcb into struct udpcb and updated the
intoudpcb macro to use __containerof to locate it. This change accidentally
introduced a dependency on the identifier inp being defined in the block the
macro is expanded in. This should have been the macro argument ip. This change
makes this simple correction.

No functional change intended.

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")