bonding: add extra field for bond_opt_value

Adding an extra storage field for bond_opt_value so we can set large
bytes of data for bonding options in future, e.g. IPv6 address.

Define a new call bond_opt_initextra(). Also change the checking order of
__bond_opt_init() and check values first.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>