Merge branch 'arp-always-override-existing-neigh-entries-with-gratuitous-ARP'

Ihar Hrachyshka says:

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arp: always override existing neigh entries with gratuitous ARP

This patchset is spurred by discussion started at
https://patchwork.ozlabs.org/patch/760372/

 where we figured that there is no
real reason for enforcing override by gratuitous ARP packets only when
arp_accept is 1. Same should happen when it's 0 (the default value).

changelog v2: handled review comments by Julian Anastasov
- fixed a mistake in a comment;
- postponed addr_type calculation to as late as possible.
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Signed-off-by: David S. Miller <davem@davemloft.net>