bonding: Allow setting and querying xmit policy regardless of mode

From: Wagner Ferenc <wferi@niif.hu>

For consistency with the behaviour of the arp_ip_target option,
let /sys/class/net/bond0/bonding/xmit_hash_policy accept and report
current policy even if the bonding mode in effect does not use it.

Signed-off-by: Ferenc Wagner <wferi@niif.hu>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>