net: convert /proc/net/rt_acct to seq_file

Rewrite statistics accumulation to be in terms of structure fields,
not raw u32 additions. Keep them in same order, though.

This is the last user of create_proc_read_entry() in net/,
please NAK all new ones as well as all new ->write_proc, ->read_proc and
create_proc_entry() users. Cc me if there are problems. :-)

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>