net: vxlan: make vxlan_set_mac() return drop reasons

[ Upstream commit d209706f ]

Change the return type of vxlan_set_mac() from bool to enum
skb_drop_reason. In this commit, the drop reason
"SKB_DROP_REASON_LOCAL_MAC" is introduced for the case that the source
mac of the packet is a local mac.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 1f5d2fd1 ("vxlan: Fix NPD in {arp,neigh}_reduce() when using nexthop objects")
Signed-off-by: Sasha Levin <sashal@kernel.org>