[NET]: Make some network-related proc files use seq_list_xxx helpers

This includes /proc/net/protocols, /proc/net/rxrpc_calls and
/proc/net/rxrpc_connections files.

All three need seq_list_start_head to show some header.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>