Fix spa config generate memory leak in spa_load_best function

When spa retry load succeeds and spa recovery is requested it may
leak in spa_load_best function. Always free the generated config
when it is not assigned to the spa.

Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4940