bonding: fix compile error if debug enabled

This is what I get if debug is enabled:

drivers/net/bonding/bond_ipv6.c: In function 'bond_na_send':
drivers/net/bonding/bond_ipv6.c:75: error: 'slave' undeclared (first use in this function)
drivers/net/bonding/bond_ipv6.c:75: error: (Each undeclared identifier is reported only once
drivers/net/bonding/bond_ipv6.c:75: error: for each function it appears in.)

This patch fixes that.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: David S. Miller <davem@davemloft.net>