bonding: use nla_get_u64 to extract the value for IFLA_BOND_AD_ACTOR_SYSTEM

bond_opt_initval expects a u64 type param, it's better to use
nla_get_u64 to extract the value here, to eliminate a sparse
endianness mismatch warning.

Fixes: 171a42c3

 ("bonding: add netlink support for sys prio, actor sys mac, and port key")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>