storageproxyd: Add missing fsync after symlink creation

The recently added com.android.storage-unittest.td-init-check-clean
revealed a missing fsync after creating the symlink when using the
--file_storage_mapping option.

Bug: 34764185
Test: --test com.android.storage-unittest.td-init-check-clean
Change-Id: Iddd8a149040e21d09852d3fa68091c4ad92fdb54