ipv4: Make neigh lookup keys for loopback/point-to-point devices be INADDR_ANY

This used to be the previous behavior in older kernels but became broken in a263b3093641f (ipv4: Make neigh lookups directly in output packet path) and then later removed because it was broken in 0bb4087cbec0 (ipv4: Fix neigh lookup keying over loopback/point-to-point devices)

Not having this results in there being an arp entry for every remote ip address that the device talks to. Given a fairly active device it can cause the arp table to become huge and/or having to add/purge large number of entires to keep within table size thresholds.

$ ip -4 neigh show nud noarp | grep tun | wc -l
55850

$ lnstat -k arp_cache:entries,arp_cache:allocs,arp_cache:destroys -c 10
arp_cach|arp_cach|arp_cach|
 entries|  allocs|destroys|
   81493|620166816|620126069|
  101867|   10186|       0|
  113854|    5993|       0|
  118773|    2459|       0|
   27937|   18579|   63998|
   39256|    5659|       0|
   56231|    8487|       0|
   65602|    4685|       0|
   79697|    7047|       0|
   90733|    5517|       0|

v2:
- fixes coding style issues

Jim Westfall (2):
net: Allow neigh contructor functions ability to modify the
primary_key
ipv4: Make neigh lookup keys for loopback/point-to-point devices be
INADDR_ANY

 include/net/arp.h    | 3 +++
 net/core/neighbour.c | 4 ++--
 net/ipv4/arp.c       | 7 ++++++-
 3 files changed, 11 insertions(+), 3 deletions(-)

2.15.1