neigh: enable garbage collection

[dirkmueller]

VIPs and floating ips that move between differnet interfaces might stay for very long times cached incorrectly in the neighbor table until the garbage collection kicks in. by default a STALE (so an entry that used to have an active connection but now doesn't anymore) gets garbage collected after gc_stale_timeout, but only if there are more than gc_thresh1 STALE entries in total. The default of 128 means that one has to accumulate 128 stale entries (or trigger a forced cache flush) until this is happening, which for small/low traffic clouds can take an eternity.