net: bridge: pack net_bridge better

Further reduce the size of net_bridge with 8 bytes and reduce the number of
holes in it:
 Before: holes: 5, sum holes: 15
 After: holes: 3, sum holes: 7

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>