[NEIGH]: Make neigh_add_timer symmetrical to neigh_del_timer.

The neigh_del_timer() looks sane - it removes the timer and
(conditionally) puts the neighbor. I expected, that the
neigh_add_timer() is symmetrical to the del one - i.e. it
holds the neighbor and arms the timer - but it turned out
that it was not so.

I think, that making them look symmetrical makes the code
more readable.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>