[NET]: Fix ifenslave to not fail on lack of IP information

Patch to ifenslave so that under older ABI versions, a failure to propogate ip
information from master to slave does not result in a filure to enslave the
slave device.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>