udp: improve handling of cached route

The inp_route pointer should only be provided to the network
layer, when no destination address is provided. This is only
one of the conditions, where a write lock is needed.
If, for example, the route is also cached, when the socket is
unbound, problems show up, when the sendto is called, then
connect and finally send, when the route for the addresses
provided in the sendto and connect call use different outgoing
interfaces.
While there, clearly document why the write lock is taken.

Reported by: syzbot+59122d2e848087d3355a@syzkaller.appspotmail.com
Reviewed by: Peter Lei, glebius
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46056