Add support for flush VXLAN FDB entries by destination VNI. FDB entry is
stored as {MAC, SRC_VNI} + remote. The destination VNI is an attribute
of the remote. For multicast entries, the VXLAN driver stores a linked list
of remotes for a given key.

In user space, each remote is represented as a separate entry, so when
flush is sent with filter of 'destination VNI', flush only the match
remotes. In case that there are no additional remotes, destroy the entry.

For example, the following are stored as one entry with several remotes:
$ bridge fdb show dev vx10
00:00:00:00:00:00 dst 192.1.1.1 vni 3000 self permanent
00:00:00:00:00:00 dst 192.1.1.1 vni 4000 self permanent
00:00:00:00:00:00 dst 192.1.1.1 vni 2000 self permanent
00:00:00:00:00:00 dst 192.1.1.2 vni 2000 self permanent

When user flush by VNI x, only the relevant remotes will be flushed:
$ bridge fdb flush dev vx10 vni 2000

$ bridge fdb show dev vx10
00:00:00:00:00:00 dst 192.1.1.1 vni 3000 self permanent
00:00:00:00:00:00 dst 192.1.1.1 vni 4000 self permanent
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>