ipv4: reintroduce route cache garbage collector

ipv4: reintroduce route cache garbage collector

[ Upstream commit 9f28a2fc0bd77511f649c0a788c7bf9a5fd04edb ]

Commit 2c8cec5c10b (ipv4: Cache learned PMTU information in inetpeer)
removed IP route cache garbage collector a bit too soon, as this gc was
responsible for expired routes cleanup, releasing their neighbour
reference.

As pointed out by Robert Gladewitz, recent kernels can fill and exhaust
their neighbour cache.

Reintroduce the garbage collection, since we'll have to wait our
neighbour lookups become refcount-less to not depend on this stuff.

Reported-by: Robert Gladewitz <gladewitz@gmx.de>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>