net: vxlan: make vxlan_snoop() return drop reasons

Change the return type of vxlan_snoop() from bool to enum
skb_drop_reason. In this commit, two drop reasons are introduced:

  SKB_DROP_REASON_MAC_INVALID_SOURCE
  SKB_DROP_REASON_VXLAN_ENTRY_EXISTS

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>