bond: Use correct type for slave's change_seq.

bond: Use correct type for slave's change_seq.

seq values are 64-bit, and storing them to a 32-bit variable causes
the stored value never to match actual seq value after the seq value
gets big enough.

This is a likely cause of OVS main thread using 100% CPU in a system
using bonds after some runtime.

VMware-BZ: #1564993
Reported-by: Hiram Bayless <hbayless@vmware.com>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>