vxlan: Add missing VNI filter counter update in arp_reduce().

VXLAN stores per-VNI statistics using vxlan_vnifilter_count().
These statistics were not updated when arp_reduce() failed its
pskb_may_pull() call.

Use vxlan_vnifilter_count() to update the VNI counter when that
happens.

Fixes: 4095e0e1

 ("drivers: vxlan: vnifilter: per vni stats")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>