ipmr: missing dev_put() on error path in vif_add()

The other error paths in front of this one have a dev_put() but this one
got missed.

Found by smatch static checker.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Wang Chen <ellre923@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>