vxlan: Use FDB_HASH_SIZE hash_locks to reduce contention

The monolithic hash_lock could cause huge contention when
inserting/deletiing vxlan_fdbs into the fdb_head.

Use FDB_HASH_SIZE hash_locks to protect insertions/deletions
of vxlan_fdbs into the fdb_head hash table.

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Litao jiao <jiaolitao@raisecom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>