net: make u64_stats_init() a function

Using a function instead of a macro is cleaner and remove
following W=1 warnings (extract)

In file included from net/ipv6/ip6_vti.c:29:0:
net/ipv6/ip6_vti.c: In function ‘vti6_dev_init_gen’:
include/linux/netdevice.h:2029:18: warning: variable ‘stat’ set but not
used [-Wunused-but-set-variable]
    typeof(type) *stat;   \
                  ^
net/ipv6/ip6_vti.c:862:16: note: in expansion of macro
‘netdev_alloc_pcpu_stats’
  dev->tstats = netdev_alloc_pcpu_stats(struct pcpu_sw_netstats);
                ^
  CC [M]  net/ipv6/sit.o
In file included from net/ipv6/sit.c:30:0:
net/ipv6/sit.c: In function ‘ipip6_tunnel_init’:
include/linux/netdevice.h:2029:18: warning: variable ‘stat’ set but not
used [-Wunused-but-set-variable]
    typeof(type) *stat;   \
                  ^

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>