Fix memory leak in function add_config()

Config of a hot spare or l2cache device will leak memory in function
add_config(). At the start of this function, when dealing with a
config which belongs to a hot spare not currently in use or a l2cache
device the config should be freed.

Signed-off-by: liaoyuxiangqin <guo.yong33@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4910