bonding: disallow change of MAC if fail_over_mac enabled

Currently, if fail_over_mac is set to active, then attempts to
change the MAC of the bond itself silently fail.  However, if fail_over_mac
is set to follow, changes are permitted.

	Permitting the bond's MAC to change with fail_over_mac=follow
will disrupt the follow functionality, which normally controls the
assignment of MAC address to the bond and its slaves, and can cause
multiple ports to be assigned the same MAC address. which will interfere
with the functioning of the device (where the device here is a
virtualization-aware card for s390, qeth).

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>