[ATM]: Fix clip module reload crash.

net/atm/clip.c crashes the kernel if it (module) is loaded, removed,
and then loaded again.  Its exit call to neigh_table_clear()
should destroy the cache after freeing it.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>