net: pfcp: allocate per-cpu tstats for PFCP netdevs

In the Linux kernel, the following vulnerability has been resolved: net: pfcp: allocate per-cpu tstats for PFCP netdevs PFCP uses dev_get_tstats64() as its ndo_get_stats64 callback, but pfcp_link_setup() does not request NETDEV_PCPU_STAT_TSTATS. The net core therefore leaves dev->tstats NULL for PFCP devices. Creating a PFCP rtnetlink device can immediately ask the new netdev for stats while building the RTM_NEWLINK notification. That reaches dev_get_tstats64() and dereferences the NULL dev->tstats pointer. Set pcpu_stat_type to NETDEV_PCPU_STAT_TSTATS during PFCP link setup so the net core allocates the storage expected by dev_get_tstats64().