neigh_add_timer cannot use add_timer unconditionally. The reason is that
by the time it has obtained the write lock someone else (e.g., neigh_update)
could have already added a new timer.
So it should only use mod_timer and deal with its return value accordingly.
This bug would have led to rare neighbour cache entry leaks.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
6fb9974f49