Eliminate use of the global ripsrc which was being used to pass address
information from rip_input() to rip_append(). Instead, pass the source
address for an IP datagram to rip_append() using a stack-allocated
sockaddr_in, similar to udp_input() and udp_append().

Prior to the move to rwlocks for inpcbinfo, this was not a problem, as
use of the global was synchronized using the ripcbinfo mutex, but with
read-locking there is the potential for a race during concurrent
receive.

This problem is not present in the IPv6 raw IP socket code, which
already used a stack variable for the address.

Spotted by: mav
MFC after: 1 week (before inpcbinfo rwlock changes)