drivers: vxlan: vnifilter: free percpu vni stats on error path

In case rhashtable_lookup_insert_fast() fails inside vxlan_vni_add(), the
allocated percpu vni stats are not freed on the error path.

Introduce vxlan_vni_free() which would work as a nice wrapper to free
vxlan_vni_node resources properly.

Found by Linux Verification Center (linuxtesting.org).

Fixes: 4095e0e1 ("drivers: vxlan: vnifilter: per vni stats")
Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>