OTA: make lp_metadata write more atomic

Powerloss of OS failure occurs when super is merging,
then the device can't bring up again for no lp_metadata sync,
so make lp_metadata sync in time.

Change-Id: I1ea31662b838e35022a566b614796f2d5e05df4b
Signed-off-by: Jia Jia <jia.jia@zte.com.cn>