UDP lib inuse checks will walk the entire hash bucket to check if the
portaddr is in use. In the case of reuseport we can stop searching when
we find a matching reuseport.

On a 16-core VM a test program that spawns 16 threads that each bind to
1024 sockets (one per 10ms) takes 1m45s. With this change it takes 11s.

Also add a cond_resched() when the port is not specified.
Signed-off-by: Eric Garver <e@erig.me>
Signed-off-by: David S. Miller <davem@davemloft.net>