net: netdevice.h: sw_netstats_rx_add helper

some drivers/network protocols update rx bytes/packets under
u64_stats_update_begin/end sequence.
Add a specific helper like dev_lstats_add()

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>