bonding: fix double dev_add_pack

	Bonding can erroneously register the same packet_type to receive
ARPs (for use by ARP validation): once at device open time, and once via
sysfs.  Since sysfs can change the validate setting (and thus register
or unregister) at any time, a flag is needed to synchronize with device
open in order to avoid double registrations, and the simplest place is
within the packet_type structure itself.  Double unregister is not an
issue.

	Bug reported by Ulrich Oelmann <ulrich.oelmann@web.de>.

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>