bonding: move procfs code into bond_procfs.c

V2: Move #ifdef CONFIG_PROC_FS into bonding.h, as suggested by David.

bond_main.c is bloating, separate the procfs code out,
move them to bond_procfs.c

Signed-off-by: WANG Cong <amwang@redhat.com>
Reviewed-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>