net:snmp: Introduce generic interfaces for snmp_get_cpu_field{, 64}

This is to introduce the generic interfaces for snmp_get_cpu_field{,64}.
It exchanges the two for-loops for collecting the percpu statistics data.
This can aggregate the data by going through all the items of each cpu
sequentially.

Signed-off-by: Jia He <hejianet@gmail.com>
Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>