net/core: __hw_addr_create_ex does not initialize sync_cnt

The sync_cnt field is not being initialized, which can result
in arbitrary values in the field.  Fixed by initializing it to zero.

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Reviewed-by: Vlad Yasevich <vyasevic@redhat.com>
Tested-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>