vxlan: use __be32 type for the param vni in __vxlan_fdb_delete

All callers of __vxlan_fdb_delete pass vni with __be32 type, and
this param should be declared as __be32 type.

Fixes: 3ad7a4b1

 ("vxlan: support fdb and learning in COLLECT_METADATA mode")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>