fiemap: add fsync after write split file name

If device abnormal restart during upgrade, update_engine just
created split cow image file and wrote the name to xxx-cow-img.img.
After restart, update_engine will report following err:

06-30 08:48:06.466  root  8918  8920 E gsid    : Error removing image
product_b-cow-img: File name too long

That is because SplitFiemap::Create->DetermineMaximumFileSize
open xxx-cow-img.img to get some info, then fsync and unlink this
file.

On device use f2fs, after restart, f2fs_recover_fsync_data will
recover this file with inode block sync to ufs in following path:
DetermineMaximumFileSize
    ->FiemapWriter::Open
        ->AllocateFile
This make xxx-cow-img.img file content corruption.

Update_engine call TryCancelUpdate to clean up the residual files
of last upgrade, cannot read valid split file name from
xxx-cow-img.img, then report err log mentioned above.

Add fsync, f2fs can use correct inode to recover related files.
This can reduce the probability of this issue.

Test: abnormal restart when upgrade

Change-Id: Iceb3e5604d90495350c5fb1fb3433732372f3fec
Signed-off-by: lijiazi <lijiazi@xiaomi.com>