proc: Reduce cache miss in snmp_seq_show

This is to use the generic interfaces snmp_get_cpu_field{,64}_batch to
aggregate the data by going through all the items of each cpu sequentially.
Then snmp_seq_show is split into 2 parts to avoid build warning "the frame
size" larger than 1024.

Signed-off-by: Jia He <hejianet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>